Bison bag
I have a Bison bag that fits that space, handy for travel tools,oil etc. on the other side I riveted a canvas clip strap to the floor that wraps around my tiedown bag, fits perfectly between the seatbelt retainers and the seat back and the flap rod. Added the location to my W&B table as it helps the CG on my -7 when loaded for Oshkosh etc.
Figs