Modular 2x4 Workbench

Mark all four legs 12" up from the bottom and square the line across with the speed square. Clamp and attach the legs to the 45" rails with 2-3/4" screws — the 12" line locates the bottom shelf rail. Two identical leg sets.

Screw 21" crosspieces between the leg sets at the top, two screws per joint, forming the top frame.

Repeat at the 12" marks for the bottom shelf frame. The bench now stands; check it doesn't rock before filling in the shelf.

Find the center of the bottom shelf opening and the center of one 21" slat; match them and screw down. Then center a slat in each remaining opening, and repeat — every gap self-equalizes, no measuring, no cumulative error.

Steal this method for every slatted thing you ever build.

Set the plywood panel on top, adjust the base until the panel's edges match the frame, and screw down with 1-1/4" screws about every 12". The factory panel edges square the bench as a bonus.

Going mobile: measure your casters' installed height, cut exactly that off the legs, and screw the casters to the leg bottoms. Same bench, same height, now it follows the work.