I have been putting off fabricating the attachment for the plywood avionics shelf, but the time has come since I need the shelf in place to mount the heater. For the forward side of the shelf I drilled into the lower flanges of the firewall cross member. This was a bear to do since I needed to get at it from underneath. The aft side was easier. For that I riveted an L angle to the cabin frame cross member and drilled holes in the top side of the L for screws to attach to the shelf. Of course all this meant repitions of removing and replacing the shelf, so all the wiring got disconnected. Once the shelf was in and attached (hopefully for the final time) I mounted the heater using high temp grommets from Grainger and the threaded rods I prepared earlier. The heater seems to be rock-solid in place. Finally, I reconnected all the wiring again and finished up a few additional connections for the heater.
It was a long day and not much visible progress was made, but these things needed to get done to continue. It was a lot of tough angles to contort my middle aged overweight body into and I have the cuts and scrapes to prove it.
Heater mounted with high temp grommets
Aft side of shelf screwed to L angle riveted to cross bar
Passenger side shelf attached to firewall cross member and heater mounting top.