Cut an approximate-sized mounting plate from 0.050" aluminum, then fabricated four angle attach points that will attach the transponder tray. I laid out the original strobe mounting plate, and transferred its nine original screw locations onto the new plate. Next, after a bit of pondering and taking measurements in the aft fuselage, I traced the outline of the transponder tray into what should be the best location on the mounting plate--one that will allow access to the mounting screws and also give enough room for the wiring harness.