I think for the cards, if you don't have a spare AT case to rip one from, I would probably take a 1/4" plexiglass and make it the length it needs to span the AGP and PCI cards. Then, using a reference, I would dril holes after the cut. In place of my metal bracket, I would find some good epoxy (for this case, I had 2 ton epoxy left over) and glue it with the back part of the case at the right height and all.
I think a good inch by six and a quarter piece at 1/4" would to glue it in a fashion that part of the piece would be glued to the case itself.
If you want the removable motherboard routine, I would opt for square dowels at 1/2", adjusting how the dowel's thickness should be because of the card connectors, and build a backplate. I would measure everything precisely and accurately so that the cards would make a good fit. The dowel should be at a motherboard's length and you should have more dowels making up the rest of the backplate frame. Then, you can glue a board to the backplate frame with holes (or in my case, I used drilled dowels) for standoffs.
I have included a
picture for both scenerios. However, if you could, find a junk AT part and take out the card slots. This would be easier in my opinion.
Good luck!
🙂