Oh Derek... you and your wheel reinventing!
Rather than create potential problems in the future, why not go the traditional/obvious route and use square nuts fitted into appropriately-sized slots beneath the bench top?
Most decent fastener suppliers carry square nuts. Antique beds were all held together at the corners with 3/8" or 1/2" bolts through the legs and square nuts in slots cut into the rails. I've had occasion to use a rattle gun to undo stuck bed bolts and the square nuts always stayed put in their slots.
If you can't find square nuts locally, they're easily made from a length of 1" x 3/8" (25 x 10) flat bar...
I know you believe you understand what you think I wrote, but I'm not sure you realize that what you just read is not what I meant.