Quandary

I completed these three mixed media paintings several months ago.  They are large....19 x 23 inches.  The base is heavy bristol paper and I've painted and collaged the surface.  There are some slightly dimensional elements too.  Here is the quandary..... how do I mount them for display?  I can't afford to frame them because they are so big.  They need a deep frame because of the dimensional elements.  I've been reading about mounting works on paper and the experts seem to think that gluing them down to a support devalues the work.  I don't even know if these are good enough to worry about devaluing them!  There is new art district developing in my neighborhood and they are putting out invitations for artists to display their work.  These three pieces are among my few finished works of art and I'd like to see if they would be accepted for display at one of the venues in our new art district but before I can do that I have to get them ready for display.  I'm considering gluing them to masonite but then I wonder if I should use some kind of varnish to protect them?  They have a few layers of gloss medium already but some of the collage elements are just plain printer paper which is not archival and I'd like to protect it as much as possible.  What is an affordable and reasonably professional way to finish these pieces?