Roger:
That wood buck was routered with a 3/4" rounding bit, and everything sanded smooth. I did not anneal the piece because I wanted no distortion in
the flat top piece on top. I slowly beat the edges down working the corners into themselves. You cannot allow the wrinkles you form to over lap them selves.
These wrinkles are used to tuck shrink the metal into itself. Start at the top of the tuck (wrinkle) working down to the wider point to flatten it into itself.
I'm sliding the hammer towards the center of the corner as I hit the tuck. If the sides were any longer I would not have been able to control the corners.
With the .063 alum I was able to metal work the marks out, and sand it smooth. We went from 220 grit all the way to 2500 to prep for polishing.
It's very important to keep the piece clamped tightly while working, and have the cover piece as close to the edge as possible, yet give you room to hit the edge
with your body hammer. I try to use plastic mallets on the sides, but the corners require a lot of force so I use the body hammer. I order most of my alumn
from McMaster Carr which will ship 3 x 4 foot pieces ups.
Hope this helps, Rich