OK. Here's some pictures I took this morning. This first one shows a full length coyote stretcher.
Here's a close-up where the two boards meet. You don't have to wire these together. The pressure from the pelt will hold them together, but I just prefer to have mine attached.
Here's how I have them attached at the bottom. The nylon insert nut is permanently attached to the tailboard. The other two are attached to the side boards with wing nuts so I can loosen them up if the pelt is hard to get off the board.
Mark the lengths on your board where the pelt will go up...or down in size. Furs are graded by size from the tip of the nose to the base of the tail. If the pelt is within an inch of grading up to another size you can often stretch it enough to get the extra length. Don't over stretch though because you'll cause the flanks to look thin and that'll cost you $$.
Some red fox and pine marten boards. These will need belly wedges because they're solid boards. If the pelt gets stuck it'll be a booger to get off.
Belly wedges are just tapered boards that slide between the pelt and the stretcher. I make mine from the scraps I have left over after making the stretchers.
I think this should pretty much get you started. You did mention "adjustable" stretchers in your original post. I would recommend you use these standard type stretchers. As I said above furs are graded by size and most buyers want the widths to be pretty standard. Sometimes you'll have an animal that just doesn't fit on these standard boards and you might have to make another one just for that size. I have bobcat boards in four different sizes just for that reason.
If you still want to make them adjustable, don't make the tail boards as wide as I have in the above picture and instead of using bolts and washers on the side boards at the bottom use dry wall screws so you can adjust them easily or you can stick with the bolts and nuts and cut slots in the board so they'll slide to the width you want and then you can tighten the wing nuts.
Mike