A German company did one for the military, but not available to the GA market. A number of people do boxes with a transponder in, normally including a battery, for balloon pilots. Try the balloon community.
It wouldn't be difficult to fit a TRIG TT21 in a Turb, plus a 10AH LiFePO4 battery pack (or SLA if you don't fill the tank/go on a diet etc.).
You wouldn't even need to have the control unit on the panel, just leave it on 7000, 1200 or whatever is the generic VFR squawk locally.