I know spar varnish is typically recomended for outdoor furniture, etc., but I've read that it was initially used for sailing vessels, (hence the "spar") and flexibility was more important then durability (they were repainted/coated on a regular basis).
You should be able to find an epoxy mix in the same section - something along the lines of enviro-tex lite, pour-on, etc. Usually advertised for making those bar/table tops with trinkets and whatnot embedded in them. I've used the stuff for quite a few paludariums and backdrops and such (nothing currently in my possession).
It's going to be more expensive then spar varnish, but should still be reasonably priced, it's relatively easy to work with (use small batches), and has minimal odor/fumes.