[Van]

A technicque I've used in the past that works really well is this;
1. Attach the caster to a square of 3/4" plywood. I prefer to use an
1-1/4" Stove bolt with the head counter-sunk into the top of the plywood. Be sure to use a split washer to lock the nut in place.

2. You can now attach the wooden square to the bottom sie of the platform with Drywall screws. Again 1-1/4" work well, so you don't go all the way through the top.

The nice thing about this system is it's really fast when you needto pull things apart and/or re-attach them.