I just finished a foam and drylock background for my 20 H and have pix I'll try to post here and on the 75 g DIY background thread so people can see...Drylock is a bit thicker than paint, but even with 3 coats of the stuff it didn't obscure the detail in my build. Can't wait to see how yours turns out!
For those who have not done these, it's pretty easy (but messy)--foam, silicone (GE I is safe), cutting/rasping/shaping tools, drylock, concrete colorant, and your imagination are pretty much all you need!