Going to mount the transponder behind the sub-panel but need to make some supports to hold the tray. Planning to mount the tray on the copilot side and allow room for the transponder to slide out on the pilot side. I had previously ordered the tray from Steinair. Used the tray to fabricate some 1 1/2” angles to span between the forward ribs. Cut the angles to length and then ripped one leg down to about 7/8”. Marked the holes from the tray and drilled those mounting holes. Also marked out the airflow holes in the tray to provide holes in the angle as well. And drilled a few more holes just for lightening the thick angles. Came out well. Then clamped the assembly behind the sub-panel and match drilled holes to the forward ribs for AN3 bolts. It will be tough, but in theory the whole tray should be removable. Transponder itself should be easy slide in/slide out.