I posted it, but I lost the pics.
Drill 3" blind holes in the edge of a 2X4. Make them as snug as practical. For 16ga, a 3/4 spade bit works great. For a 12ga, which I haven't done, a 13/16 would be ideal; 7/8 might be too sloppy.
Insert the hulls in the holes and put the case gheads against the fence. Set the fence to rip at the desired OAL, including the case rim thickness. With block loaded with hulls, make your rip cut. Hold the case heads snugly against the fence as you feed the board and I doubt vibration will be a problem. Remove the freshly cut hulls and reload.
Again, the goal is to bore the holes minimally oversize. Ripping the board thru the holes would be an alternative, but clamping the halves while sawing could be problematic. If I wanted a split jig, I'd split it such that I could use bolts and wingnuts to close it and not interfere with the blade and fence.
