I decided to mount the transponder antenna diagonal from the comm antenna to get the 3+ feet separation needed to prevent interference. I drilled a hole in the fuselage floor on the passengers side about 2 inches from the fuel vent bulkhead fitting and screwed the antenna in place. No need for a doubler plate here either given the small size of the antenna and how stiff the floor is in that front corner just behind the cowl.
Steven helped me tighten up the nuts on the antenna as well as the fuel vent fittings. He worked from the outside while I crawled up underneath the panel on the inside.
