I think the place to start is to buy a set of stretcher bars and study it first hand until you can visualize the joint. Here is a very inexpensive set on Amazon for that purpose https://www.amazon.com/Masterpiece-S...stretcher+bars
. While there, check for large quantities of standard sizes, it is probably as cheap as making your own or standard sized canvases.
I would like to make these in batches, since most of my wife's paintings are 16x20, but since this is an end joint, once you master it, the "rails and stiles" can be any length. Looks like everything is in 1/8th increments with a little plane or router work to put an edge to set back the canvas from the frame. I'd want to work out some sort of jig to space the blade and fence precisely, and use a standard procedure to make all the same cuts at the same time. This could be a simple stack of spacers. I see that this is a loose joint and that wedges do the stretching work.
You'd apply the stretched canvas using a canvas stretcher tool then do more stretching with water, and then finally, re-stretch with the wedges. Some of the spacing may depend on the width of the frame, or perhaps for oversized canvases, the thickness.
I'm going to go ahead and follow my own advice and order a sample now. The video above was very helpful on placing the wedges. I think it is more economical to order batches of prefabricated pieces for standard sized paintings, and save making for oversized and odd sized canvases.
Thanks for posting this string, I have a long roll of medium weight canvas I have just been hesitating to start using over this very issue. Here are a couple of links for canvas stretcher bars, at least a place to start. Stretcher Bar Warehouse: High Quality - Wholesale Pricing https://imageworkssupply.com/stretcher-bars.html BEST Artists' Stretcher Bars and Cross Braces - Jerry's Artarama
I suspect by the time you buy, mill and cut your own you're out more than these in bulk will cost. Hand made for large sizes makes some sense, but not necessarily for standard sizes.