Interesting! I had forgotten about this. When I sat in the demo Kitfox at Oshkosh in 2000 I remember feeling how the metal along the bottom edge of the doorframe seemed to flex a lot more than I thought it should when I put my weight on it to get into the cockpit. I was not bold enough to take a welding torch to my new fuselage structure but I did fill in the triangles with 1/8" plywood and structural epoxy to beef it up and it has survived some pretty, ah ... firm landings.
SeatReinforcement.jpg