I think an enterprising person could make one using a turnbuckle, a bolt, couple of wrist pins, teflon washer, wingnut, and a round headed screw at the end to act as the slot. I'm going to try and make one.
Here is what I was thinking, hopefully it'll give you an idea of how to make one.
With the turnbuckle, you'd remover the eye bolts that comes with it as you won't need them. Screw the (appropriately size/threaded) round headed screw in one end. Just above that (in the turnbuckle, but missing the screw) drill a hole for a wrist pin to be driven into. At the other end of the turnbuckle, drill the threads out just large enough for your long bolt to fit in. That longer bolt should fit all the way to the end of the turnbuckle and extend past the other end enough to get a wingnut on. Before inserting the bolt, put your wingnut on your bolt, then put your teflon washer/washers on, then insert your bolt into the reamed out hole. Drill a hole through the end of the bolt and tap a wrist pin into it. Voila! You now have a poor man's version of the ClampTite.
Anyhow, I think that would work and turnbuckles come in all sizes. Though you want to use one where the sides are open so the wrist pin that goes through the bolt can slide the length of that slot.
Here is a link with better pictures of how that thing works.
ClampTite, how it works.