These Corner Brackets Won't Hurt Wood
Some band clamps come with four metal corner brackets that can dent your project if you apply too much clamping pressure. For a kinder, gentler corner bracket, try building your own as shown in the drawings above.
Cut the center portion of the brackets out of softwood 2X material with a 2 1⁄4 " holesaw. Make the top and bottom pieces from 1⁄8 " hardboard or 1⁄4 " plywood. You can customize these blocks by cutting the notches at different angles for projects with more than four sides.
—from the WOOD® magazine shop