I’ve had similar thoughts. One idea I had was a small lockable box mounted on short running board in front of rear fender. Somehow locked onto the running board also but none of the locking mechanism visible when the tool box is removed. Could be as simple as spring loaded J Hooks on the box that push down and twist under lip of running board then another bar across the top of box to lock it all together.

1957 Chevy 3200
Daily Driver
PS, A/C, Tilt column, Rebuilt 350, Rebuilt TH350, Reupholstered Bench Seat, sound proof/insulated, LED headlights/taillights/backup lights/interior courtesy lights.
Follow in the DITY