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This how to is one that I have used twice, once on a 31 Desoto coupe which I had no trunklid for, and once to create a steel louvered and operational trunklid in a glass 27 T Ford roadster. It is not really difficult, and if you have anything other than a Ford, it may be your only choice if your car came without a trunk lid. It works very well for cars up to and including 1932---once you get beyond 1932 there is too much compound curve in the trunklid to use this method.
So...........whats the method?
I have found that all early coupes have a slight compound curve to the trunklid---that is to say, they curve a bit from side to side, as well as the obvious curve from front to rear. I start by using 4 large sheets of cardboard, setting them one at a time into the trunk opening, and tracing the profile of that particular side onto the cardboard. I then take 1" x 1" x 1/8" wall square tubing (mild steel), cut 4 peices about 12" longer than will be needed (this is important) and taking them to a metal fabricating shop that has a large set of plate rollers. I have them roll the individual peices untill they match the 4 profiles drawn onto the cardboard. It is important to have the peices cut about 12" longer than necessary because the first and last 6" on each peice will not get bent, but is used to start feeding into the plate rolling machine.
Forget something Brian?
Once I have the 4 peices rolled to match the cardboard profiles, I drill two 3/16" holes thru the side of each peice, and through the side of the opening it is matched with. I install the peices with a 3/16" thick flat washer and a sheet metal screw thru them into the side of the trunk opening. The washer is what is going to ensure a uniform gap all around the trunk edge. I miter cut the ends of each peice at 45 degrees.
step 2 in the process---do the two sides first as shown---
Third step----add in the member for the top of the opening
And finally the peice which goes along the bottom of the trunk opening. Be very particular about the 45 degree miter cuts on the ends, and put a good deep 45 degree chamfer all around the mitered ends to hold the weld bead---
Next step---while everything is securely screwed into place, cut 4 peices of the same tubing (straight peices this time, no curve required) about 10" long. Miter cut the ends at 45 degrees and install them as shown. Get out the welder of choice and tack everything that you can---don't get weld on the washers. Put enough tacks on that puppy that when you remove all the screws and take the frame out of the trunk opening that nothing is going to fall apart on you.
Undo the screws, remove the frame, and weld everything solid. On the sides which will have sheet metal in contact, grind the welds down flush with the tube surface (thats why I suggested good deep 45 degree chamfers---we still want lots of weld left after the grinding flush process).
Cut a sheet of #20 gauge body metal, and leave enough material for a 1" overlap all around the frame. Use all the C-clamps, vise grip style welding clamps, that you have, borrow some from your buddies, and try to put a clamp about every 6" all around the edges---Don't get too crazy here---we want to pull the sheet metal skin in contact with the frame but we don't want to put dents and divots in the exposed surface. Cut a 1" square relief in each corner. Take a flat faced body hammer, and gently "persuade" the sheet metal all the way around the perimeter untill it lays flat against the edges of the frame. Do NOT try to do this all in one "go-round". it ahould be done in about 4 progressive steps, bending the flanges a bit at a time. You may have to take a "pie shaped" cut about 7/8" deep every 2 or 2 1/2" on the sides of the sheet metal in order to get it to follow the frame material without kinking. The top and bottom edges do not have such a pronounced bens, so will not need as many relief cuts (maybe none, depending on the cross-body curvature)
If you are lucky, and have a bit of talent, you will end up with this---weld the flanges to the sides of the frame BEFORE you remove all the clamps. Amazingly enough, even though it is a truly compound curve, there is enough "give" in the sheet metal to conform to most most of the cars from the twenties and early thirties without kinking or oil-canning.
In a perfect world, and with patience, it will end up like this. Of course, one point that I skimmed over is the positioning of hinges and a latch mechanism. This can be a varied as you want, but whatever method you choose, it should all be done before the skin is attached, while the bare frame can be put in and out of the opening and everything perfected BEFORE the skin goes on. The first time I did this, I just about fell down dead at how easy it was, and how well it worked. If you are carefull and take your time, there will be absolutely NO BODYWORK required on the exposed trunklid surface.---just a light skim of bondo on the edges (after you have finish ground all the welds).
This is a shot of the rear on the 31 Desoto I built. The trunklid, tailpan, and license plate coves were all hand fabricated. The top was chopped 2 3/4" and the car was channeled 6".
Tonight I dug out an old scrap book (pre digital camera) and found a good pair of pictures for a "before and after" comparison . What you see is my 27 roadster that I built about 10 years ago, with a glass body. In the one picture there is no trunklid at all--just molded in. In the other picture is a trunklid I built using the above "how to" method. I cut out the opening in the fiberglass rear deck, framed in the opening with 1/2" plywood and fiberglass, and effectively created a "trunk". I then built a trunklid exactly as described in the thread and popped about 90 louvers into it. I used a pair of light compression springs as "door poppers" and the solenoid from an old Volkswagen starter for a hidden push button latch. The solenoid was powered by a push-button inside the car that only received power when the ignition key was turned on, so no-one could open the trunk unless I was there with the key.