Not much more to it than that for the base. The
locking casters will attach to the bottom of this piece, but I put
those on last. Not sure if that made the most sense, but I didn't
want the thing rolling around while I was trying to put it together.
For the casters, I used 2 1/2" tall locking casters and some
wood screws to screw them directly into the base on each corner.
You need to make sure they are close enough to the corners so that
you can get at the locks with your feet. In hindsight, larger casters
might have been a better choice.
Proceed to the next step: rear supports...