So I made a purchase ... HD currently has an in-store "special buy" 63-gal. deck box for $40 (it's not on their site, but is like this:
https://www.suncast.com/patio-yard/deck-boxes/db6300.html?.)
It's by Suncast, but a "sub brand" of theirs. Very lightweight and so no, you can't sit on it. Has a hinged lid and wheels on one end. You can lock it, but the lock would only work if a thief forgot a screwdriver or box cutter ... the plastic seems that thin.
But for $40 I ain't complaining! It snaps together and there appear to be no major gaps. Raining today, so I'll know more about that later.
Here's a test-fit inside, for example it can hold the rotisserie ring on its end.
I also re-arranged stuff on the deck (maybe kinda like rearranging the deck chairs on the Titanic ...) and this Spring will clear out some nearby, indoor storage too.