The nice thing about your design is that you really don't have to measure anything to build it. Instead you can take one board and some leather and put it together using small cutoffs for the spacer at the bottom (along with your leather in the middle) and at the top for the jaws and the same length jaw arms. It automatically goes together, all you have to do is get the overall length where you want it. Most of the cost is in time and finish.
I built one just like it with added small magnets and a beeswax hole (for the needles), features stolen from other designs. You did a very nice job on yours! I had all the components in my shop. It is amazing how cheaply you can build things if you have $5000 worth of woodworking tools in your shop! <G> (It can be done with hand tools too, just not as fast.)
Mike Keyes