I like to adhere the cork to my backgrounds w/ clay and add a little more to fill around the edges. its easier to get plants to grow in/on, and you can pull everything out later on if you don't like it...
don't tell doug, but i'm still using the entry level formula of 3 parts dr. elsey's unscented kitty litter from petco mixed w/ 1 part peat moss ... just add water a little at a time until you get a nice workable paste and mush it into all the cracks...
if you're going to attach anything heavy, silicone is a must, but for cork, clay is easy money.
here's a link to my favorite construction how-to article for beginners. John is a member here and has great articles on his site.
Building Dart Frog Terrariums / Tropical Terrarium Construction - Frog Forum