The 2 pieces of flat stock 7" long get 1/4" hole. Stack them together, clamp and drill the hole at the same time. Use the cutoff wheel to make the slot at 45 degrees in one of them.
Lay the base down on the unmodified leg. put 2 utility knife blades 6 inches apart. Lay the slotted rib down on top of them. Lay 2 more utility knife blade spacers on top of that then lay the drilled flat stock on top of that. I might try using only one thickness on the other side. Slip a 1/4" bolt through all the holes and put a nut on the threads. Clamp and tack weld 4 or 5 spots. Remove all the spacing and weld full length on the outside of the groove(of course). Remove the blade and cut/grind/sand the top of the groove so when the blade is inserted, the exposed edge of the blade stands proud by a 1/8th to 3/16th of an inch. Grind away anything that doesn't look like a fire extinguisher quick release.
Now you need to build the quick release parts. You need 2 little pieces that welds to the base and one that welds to the blade. All three look sort of like like the outline of the state of Nevada with the bottom tip cut off. Since the pieces are so small I cut them large and cut and ground them off after welding. Weld on the outside of the groove to keep the groove clear of weld bead.
with the bolt installed, massage the lengths of the material at the quick release end of the blade to fit inside the groove of the base. Leave enough space for the blade to fall out easily after pulling the pin. Weld on the 3rd tab onto the blade. Grind off anything that looks like it doesn't belong. Put the blade in the base and clamp it together. Drill a 1/4" hole through all 3 pieces of what looks like Nevada parts. Chamfer all the edges to make it a little easier to pull the pin.