I have not had a issue with silicone drying, but have always used it on glass. I use weldbond and water and ecoearth to cover the backgrounds. Here are examples, and most are stryfoam, and or greatstuff. I do let these dry for at least 7 days if not longer. Even the weldbond takes a good time to dry.
Rough construction pics here:
http://www.kylesphotos.com/album21
Rough final picture, I will have to take one now after it has grown in, it looks much better:
Also all of these have the same background:
http://www.kylesphotos.com/photos/album23/aam.jpg
I have tested the silicon ecoearth method while I think it would work fine it seemed to peel off the foam very easy. I'm sure both are great solutions, and will look great in the right hands.
Rough construction pics here:
http://www.kylesphotos.com/album21
Rough final picture, I will have to take one now after it has grown in, it looks much better:

Also all of these have the same background:
http://www.kylesphotos.com/photos/album23/aam.jpg
I have tested the silicon ecoearth method while I think it would work fine it seemed to peel off the foam very easy. I'm sure both are great solutions, and will look great in the right hands.