If you are just looking for a 4x5 contact frame, they go pretty cheap on that auction site. 8x10's go for more because they are used allot more.
Your idea sounds sketchy to me, as far as getting enough pressure for good contact, and maintaining registration, if I'm understanding your idea correctly. The pressure needs to be even over the whole of the negative area, and matt board isn't stiff enough to do that.
Here's a post with instructions for a simple el cheapo home built frame that might work:
(there was a url link here which no longer exists)
Or you can use just plain glass weighed down, or a heavier thicker piece, but you won't be able to inspect it during exposure.