I made a no-drill metal bracket.
It's held in with 2 bolts - usng the holes for the child restraint anchors near the tailgate lip.
(All the other small bolts in the pic are just holding the bracket together)
Made it a nice tight fit -
The tub walls stop any left-right movement.
Tailgate lip stops any aft movement.
At the front, the bracket slides under seat brackets - those brackets and the seat belt mount in the center of the tub stop any forward movement.
Ran some cambuckle straps under the bracket.
Was going to use ratchet straps but the store I went to didn't have any - the cambuckle straps have held up well
Then tied the sub down.