What you wind-up with is an easy clamp that makes good solid and “square” frames and stretcher bars up to about 48 X 48″

Make corner clamps slightly thicker than what you will be clamping. Drill (2) 1/4″ holes; one for carriage bolt, the other for corner relief.

On a bandsaw, cut the square clamp area. Then “re-saw” to desired thickness.

Corner clamp re-sawed to a little thicker than frame or stretcher bar you’ll be using. I made two thicknesses; one for stretcher bars and one for gallery frames.

The clamp is made with (4) 36″ steel straps with holes, a turnbuckle for tightening, and bolts with wing nuts to hold the clamps (I used 1/4-20 carriage bolts). I got everything here from Home Depot.

Detail of turnbuckle. Note spacers cut from plastic pipe to center the turnbuckle up and down.

Number the holes on the straps for ease of stetting up for clamping. You can specially mark holes for commonly used sizes.