These clamping jigs are easy to make and you'll use them constantly. I've found that 4-inch and 6-inch jigs fill most of my needs.
Each jig consists of two L-shaped 3/4-inch MDF pieces sandwiched between a pair of metal corner mending plates (Stanley). Make sure that the MDF is slightly recessed from the long outer edges of the mending plates. Drill 1/4-inch holes through the "sandwich" on both legs for 2-1/2-inch machine bolts. Use lock washers and hexnuts to hold the sandwich together. Drill a 5/16-inch hole through each leg of the assembly about 3 inches from the corner. Insert the pin end of a Bessey TK6 fence clamp in each 5/16-inch hole, with the adjustment end of the clamp facing outward, and your clamping jig is ready to use.
Use a pair of jigs to hold box corners together for gluing and fastening. The jigs fold nicely to hang on your shop wall.