Agree with Jesse - under the rear seats makes maintenance hard.
I mounted both bent whip comm antennas along the tunnel, one in front of the other, both aft of the forward wing spar, one on the left edge of the tunnel and one on the right edge. I used a piece of joggled aluminum going over the tunnel rib flange, sharing the rib to skin rivets as a backing plate. Access is easy, just have the flap motor and control tubes installed before selecting the antenna mounting locations to avoid interference. The XPDR antenna is mounted under the co-pilot seat. The ADS-B antenna is just aft of the baggage compartment bulkhead (as is the ADS-B receiver). Marker Beacon antenna is a simple wire tie wrapped under the engine and connected to a BNC bulkhead fitting on the firewall. It works well.
Warning - these antennas are severe eye hazards for hangar dogs and pilots cleaning the airplane belly. Slide a put a piece of tubing over each antenna for protection.
Carl