Mike,
I think what you are talking about is called a scupper valve. I don't have any recommendations on them, but I think you are either taking on too much water from something that needs to be fixed, or you are just being too worried about having water in your bilge. If you really want a drain in the back, I think the style with the brass tube that you insert in the hole you drilled and then use the expanding rubber plug would work just fine, that's what mine has on it (guessing that was factory). It you don't like the idea of the rubber plug, you could get the type with the threaded brass plug and the flange screws to the transom (many stern drive boats use this style). I would be worried that a scupper valve style would fail over time and sink your boat. I believe they are designed actually for draining water that becomes trapped above deck and not below the waterline. The sea doo jet boats used them to drain the bathtub you sat in while riding in one.