Tom, I saw that and thought of you. I think if I made some I would make them out of 3/4" Russian plywood. There might be a problem with the wood splitting and that is the reason I thought plywood might be a good way to go.Just noticed a new item from Woodpecker for clamping mitered corners. This was mildly interesting since I make frames for my wife, the artist. Today, she and her daughter decided to start selling her work, and so my interest increased. The picture below shows their aluminum jigs. The drawing is of the shop made version. Eight are required. The 45 degree V cut allows you to put clamping pressure on the opposing sides of the miter cut. Clamping miters is a pain.
Since she's also going to need canvas stretchers, I decided to make a simple version of the clamping jig. Commercial frame stretchers are fairly complex, but the same thing can be done by making a simple 5/8ths frame, then cutting a spline in the corner to reinforce it. Then I can make larger canvases for her rather than buying them already made up.
So the drawing is a design I'll be making out of some nice birch I have laying around. I'll use small Bessy clamps to hold it together, they're very inexpensive at HD.
Thought some of y'all might find it handy to make a set of these for your shop. I'll be applying a little sandpaper to the side up against the frame