shower drain forSterling 48" shower surround

I have a Sterling 48" shower surround in my master bathroom. A few months after installation I noticed water leaking in my garage. I look at the drain in the base of the surround and there is an
1/8" gap between drain and base of surround that is filled with silicone caulk. I put more caulk in and seems to remain soft. I would like to replace it with a drain that will completely cover the 4.5" hole in the base. I would like the drain to fit like a drain in a tub or sink, where it overlaps the hole. I looked at wingtite and it's 4 1/8" in diameter.

Does anyone know if someone makes a drain that will cover the 4.5" hold in the shower base?
Thanks,

A 4-1/2" shower drain is a standard size that is available at any decent plumbing supplier. The problem I believe you are having is that half of the drain must be installed from UNDER the base, and only the top half screws in from above. The top and bottom halves must mate properly for the drain to seal.

Here a a picture for reference-
http://www.siouxchief.com/Resource_/ProductMedia/303/827 Installation Instructions.pdf