Actually if your building your own displays it shouldnt be too difficult to mount it in the same fashion as an invisible shelf. I would stick with foam for the bulk of the display with a 1/8-1/4" wood base. If you notch the bottom of the foam before securing the wood base you should be able to slide the display over a pair of L brackets on the wall. you can then choose to put a couple screws up through the wood base and bracket for stability if you wish. Of course depending on the size and weight of your display you may have to play around with it a bit, so Id try it with some blank material before you mount a finished piece with models!
If you have, or have access to, a compound saw or evena hand saw and miter box you can build your own frames on the cheap by hitting up the trim section@ Home Depot or Lowes. You can usually pick up some decent 1/2-1" thick trim for a few cents a foot. A couple cuts and some brad nails away and you have your own frames, if you want you can also pick up the metal biscuts they use for picture frames there to join the corners.
