Versatile Shop Cart / Workbench with Inserts

Make the base from two 1x4 @ 37.5" and two 1x4 @ 24" pieces, the 24" pieces set inside the long ones. Pocket-hole both ends of the short pieces to join them. This is the bottom of the cart.

Glue and pocket-screw two 1x3s together to form each L-shaped leg, four total, with pocket holes at each end to attach to the base and top. Fasten the legs to the base frame.

Build a second 1x4 frame identical to the base and fasten it to the tops of the legs. The cart's box is now standing.

Drop the 3/4" top panel into the top frame inset exactly 3/4" down, so a 3/4" insert will land flush. Clamp a pair of 1x2s 1-1/2" down inside the frame as a ledge to rest the panel on while you pocket-screw it all the way around — holding it at an exact reveal one-handed is a losing fight.

Nail or screw the 1/4" back on. Add the 1x3 side supports 17-1/2" up from the bottom, then a 1x2 shelf support inside each, and drop in the shelf panel, pocket-screwing its sides to the supports. Add the caster mount boards at the bottom and screw locking casters to all four corners.

Cut flush inserts at about 35-7/8" x 23-7/8" to sit in the recess. For a miter-saw insert, build up side wings to the saw's bed height so long boards ride level — those wings double as storage for offcuts. For a Kreg-jig insert, mount the jig centered with 1"-high side rails at the jig's board height, and add handles for carrying. Store the spares on the lower shelf.