Clamping picture frames during glueup has always been a challenge. My shop-built frame clamp takes away the hassle and frustration.
The key to the clamp’s versatility lies in the adjustable corner blocks. Each one consists of two rabbeted blocks you can cut from scrap hardwood or 2x4 stock.
Once the blocks are glued together, drill two holes for a couple lengths of threaded rod. The threaded rods are sized to span the frame and pass through the corner blocks. Then I use threaded knobs to adjust the clamping pressure on the frame. This drawing shows how everything goes together.