Hi Neil, I thought the same,..it is a nice fixture, but bigger would be better...Actually the loops are nicely done, too nice! The Germans stamped them out hard and cold, and if you look close, they show it!
The reason the loop bottom isn't flat is that instead of being secured in the clamp, the grooved ram should push directly on the staple ...and then, it needs a final block, with a 1.5MM rad. groove to press against at the bottom of the jig at the very last .... won't get flat until this happens?? The clamp is neat, but overkill... when the ram picks up the stock and starts to bend, any lateral movement should be over... If it does move, just make a final adjusting saddle fixture to finish, using the same ram... ... might be a two step operation!!!
.... I call this type of discovery tool making, "Taking off on the short runway!" as your initial design seems logical but has built in problems that take much more work to cure.. but you have to give it a try, and of course, you go out thru the end markers.. but your 100% better equipped when you get on the long runway, should you be lucky enough to survive the attempt!...
....It is a nice fixture, and you should be proud of the effort and result...you're close! Keep trying!...Best to all, til....lat'r....GT.....