Angle iron could work fine. Keep in mind you may want/need to put some rake into the mounting for comfort sake, typically front higher than rear. So to use your example, you may have 5" in front, 4 at the rear of your risers. You might consider having a local sheetmetal shop bend up some nice ones from sheet stock, say 14ga, once you've determined the dimensions. They'll look nicer than pieced together options and might even persuade you to paint them to match or compliment the carpet rather than add the bulk of covering them with carpet. To that thought I'd add that rather than making a "square" box type arrangement that tend to look clunky when you get to that 4" and taller range, try angling the front and back down legs just to add visual interest. It might add a few minutes of extra fab but they would look better (isn't that part of doing a custom build anyway?)

As for mounting to the floor, it's often wise to add some strength to the floor pan. One method is the use of reinforcements such as these; http://julianos.com/anchor_plate.html. But, you can accomplish just as much with a strap of steel plug welded to the floor pan, say 1/8th by 2". Whichever way you go it's a good idea to have captive nuts on one side or the other (your choice as to whether you want to run the bolt from underneath or the top)just so you aren't trying to stretch both your arms (or require two people) to hold the fasteners when you go to bolt the seat in.