Although you can purchase similar  commercial clamps, this shop-made corner clamp works just as well, can be sized to your needs, and costs little to make.

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Although you can purchase similar commercial clamps, this shop-made corner clamp works just as well, can be sized to your needs, and costs little to make. It holds box sides or frame members at exactly 90° as your adhesive sets or while you add fasteners such as dowels or pin nails.

First, cut the base and blocks as shown. I use MDF and plywood scraps. Drill a hole into the outside blocks to accommodate a furniture leg-leveling glide (no. 78072, from Lowe's, lowes.com). Use a hacksaw to cut a slot in the threaded end of the glide, as shown. Then, press a T-nut into each hole, and add a hex nut to each glide before threading the glides into the T-nuts. Now mark square alignment lines on the base and use those to position and glue the blocks to the base.

After the glue dries, apply paste wax to the clamp's surfaces to ward off glue squeeze-out. Place your box or frame pieces in the clamp and use the hex nuts to tighten the glide pads against the workpieces. If a wrench isn't available or convenient, use a screwdriver inserted through the holes in the blocks.

The version shown here works well with 34 "-thick box sides or narrow frames. You can size the clamp parts as necessary for larger or smaller frame or box pieces.
—David Cole, New Kent, Va.