First of all there are severl types of acrylic made from several different manufacturers. Plexi is a manufacturer. Lexan is only stronger because of the way it is made. Depending on the demenions and thickness of acrylic used this wil determin the type of acrylic needed. No, silicone will not work with acrylic or structural support. Silicone works when bonding acrylic to glass only because silicone has the same chemical composition as glass and bonds well to it. I would never use acrylic from a home improvement store for a couple of reasons:
-Generally it is all extruded material and not as strong as cell cast or continuous cast acrylic.
-It is way overpriced! Look in the phone book for plastic distributors in the area. You can buy a full 4X8 sheet for what you can get a half sheet (or 2-1/4 sheets) at the local store.
For overflows continuous cast material should be fine for a DIY
overflow, ask for Acrylite FF in 1/4" They will also sell the needle bottles and solvents needed for bonding acrylic. I recommend for starters to use Weld-on #3 or #4 and when set re-seal with the # 16 which is thicker and more syrup like.
Look in the DIY
forum for a sticky thread titled acrylic fabrication.