My advice is to not make things harder than they have to be.

Making your own contact printing frame (wood or metal) is fun and it's great to use tools you made yourself but it does take a good bit of time. I made mine in several sizes (all oversized) with dovetailed joints, brass hardware, etc. If you have access to a shop and know how to make them cool. Just plan on triple the time you think it will take. As far as wood vs. metal...humidity isn't going to be that big a deal with a contact printing frame.

In the meantime you should be able to get cheap-ish frames off ebay for under $20 or so. Another option is to keep using the heavy glass sandwich and make a cut-out mask from rubylith. I've used all three options with success. The last one is by far the simplest.

The important thing is to have fun and it sounds like you have that covered.

Alan.