Maybe you have all seen this before but this website is AWESOME! I have made a few things from it and am super happy with the results. It is a great recipe for making hides, backgrounds, bowls etc! I have had no issues with my background in my balls tank getting soggy even though I used modge podge (though I used the outdoor version) but after having a killer success with the first background I ordered the Shields-All for future projects and plan on resealing my first project. Enjoy!!
http://www.lizard-landscapes.com/
P.S.
I keep rhacs too (which are high humidity) and I haven't tried this DIY in their tanks yet.