Looking for plans on how to build a jig is like looking for Solomon's treasure. Your best bet is to study every picture and document you can find and try to build your own. I would encourage you to find a small pan-am duplex jig from Goldfield or IRD, scale the size up and modify to your desire. We build them and believe it or not the most expensive piece is the diaphragm. But you can get away with using a 4 ply tire cut in half which actually outperforms the molded rubber one. Go ahead and PM me and I will try to help. BTW after using jigs for the past 40 years we much prefer round jigs to square hutch types, especially if you are going after diamonds.