Joe, I have used great stuff/coco on a couple of my acrylic enclosures. I like the way that it looks and seems to work out fine. I bought some of that waterproof expanding foam and will try that on the next one that I set up. I'll offer you two cautions: Mine took a LOT longer to cure than when I do the same thing in a vertical 15 gallon tank. I cannot explain why, but they had a "plastic" smell for about 8x as long as a glass tank does. I concur with Melissa's observation, in fact based on my experience, she gave a conservative estimate. The second thing is: you have to be a lot more careful with the silicone in one of these enclosures because you cannot use a razor to scrape off the extra. I tried using a plastic putty knife and that does not work either, it takes of the majority of it but a thin layer remains. Even after extensive rubbing of the area with a dry and wet cloth, it seems that silicon leaves a slight haze where it cured on the acrylic. I can attach a picture of one of my enclosures, unfortunately I cannot seem to get a really good picture of them.
Since you have asked this question, does that mean that you have received the enclosures from Mark Pepper? His designs look great.