In a large volume laid at one time, it'll take the center meat longer to try, so I would think about doing it in layers, this will give you more control too since it expands somewhat. Once it dries it's no-toxic, but when it's wet, it's sticky as CRAP so wear gloves if you think you're gonna be man-handling it.
Chondubrid mentioned it's water tight when it dries. I cant really attest to this, but the reason I suggested coating it in something is because its a banana yellow color, and makes for a goofy decoration alone. Coat it in SOMETHING to hide the actual product used. Also, if you're gonna cut into it after the fact, wait until it's completely dry and I found a breadknife works really well. The sharp serrated edges are much better than a flat blade. Once you've cut into it, you'll see the inside resembles bread, very porous, so this is another reason I suggested coating the surface to make it a little smoother for your water flow.
Take some pix and post them if you can!
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Tom
TCJ Herps
"The more people I meet, the more I like my snakes"