Cut eight 2-foot sections of lumber with the circular saw. Lay two sections on the floor in parallel.
Lay two sections on top of the first two at right angles, so that they make a square with 4 square inches of overlap at each corner.
Lay casters upside-down on each joint. Use the caster flange mount holes to mark screw positions.
Remove the casters and drill pilot holes with the 3/32-inch wood bit in the wood.
Drive 3-inch screws through caster holes and both 2-inch by 4-inch lumber sections. Turn the dolly over.
Set two lumber sections on end at the center of a lower cross-section of the dolly. Align them edge to edge with 2-1/2 inches of space between them.
These will form the uprights to hold the door in place. Trace pencil lines around their bases.
Repeat at the other end of the dolly. Remove the uprights for now.
At the center of each tracing and 2 inches apart, drill holes with the 7/16-inch spade bit. Stand the dolly on end.
One by one, hold an upright in position within each tracing and use the holes you just drilled as guides to mark center points on the upright ends.
Drill pilot holes into the upright ends at the marks, using the 1/4-inch spade bit. Insert each lag screw through a washer, upward through the bottom of the dolly holes and into their respective uprights.
Tighten with a wrench.
Cut carpet scraps large enough to wrap around the dolly between the uprights and staple at the bottom of the dolly. Wrap the tips of all the uprights with carpet scraps and staple at the outside edges.
Slide a door onto the dolly between the gap in-between the uprights.