Lights and top?
The Red can of GS expands alot, so use it judicially. You're going to need way more silicone than you have to cover the background. For the price of silicone, I'll seriously check out the Parts forum for the flevapol/concrete binder methods of doing backgrounds. There has been alot of activity on the topic. I did my first one last night and it's sooooo much easier than using silicone it's not even funny, and it's very inexpensive.