Personally I use a CED 8000 with a CED stop plate. It is overkill for our purposes, but is the only timer that CED offers that can accept a stop plate. They do offer a CED 7000A which is their shot timer designed specifically for airsoft. Unfortunately it doesn't have an input for a stop plate, but the microphone will pick up airsoft shots.
Madbull makes a timer and stop plate combo, but the stop plate will wear out VERY quickly. The metal on the plate is cheap and you end up shooting holes through it. Dusty and the4thpower3 had this happen to them. Here's a pic.

BAM Airsoft I believe also makes a stop plate which seems to hold up to repeated use.