Chuck:You would most likely want to hold the clamp upside down instead of like that so that your boards would be lying flat on the bench. But I guess that doesn't make as nice a photo as holding it that way. I have some like these but not Besseys: Bessy Corner Clamp | Rockler Woodworking and Hardware . They are better than holding the boards by hand but to use it easily would require two people I think. I have to fiddle with the fit until I get it right and then it's good to go. The problem is that you need one hand for each board and one to tighten the clamp. I suspect that quick grip clamp might work the same. $7 is an excellent price. I paid more than that for the other type.
Rick I think the one that Theo posted would be much easier to use than the type I posted and maybe better than the quick grip one too. One of the issues with those clamps is that when you tighten it causes the boards to move in relation to each other as the clamp tightens on both at once. That's the fiddling around part I mentioned. It often takes me several tries to get the fit right AND be clamped. With Theo's jig you can tighten one side dead flush first and when you clamp the second side it's just a matter of evening up the corners before you pull the second clamp tight. And since the clamp is pulling straight against the joint at 90* it won't tend to skew the joint like the other one does. I think I'll go out and make a set of those and my old corner clamps might be for sale too.That’s actually I neat idea to. Be a pita to set up imo, but would get the job done