It's sometimes interesting to look at patents for ideas. In the '50s and '60s truck bed covers were more popular than tool boxes and there are certainly some interesting versions.
https://patents.google.com/patent/US3420570A
https://patents.google.com/patent/US3069199A
https://patents.google.com/patent/US3012814A
https://patents.google.com/patent/US2886375A
This one is for a pull out drawer, vintage 1955.
https://patents.google.com/patent/US2784027A
This one actually converts a stepside bed into a utility box.
https://patents.google.com/patent/US3326595A