I tried some actual baits instead of cheating with a section of sprue.
The 2 1/2" fry bait screwed right on but the tail section had to be swung under the hook several times to get it fully mounted. This was the 1/16 ounce.
The 3/32 ouncer got the 2 1/2" Fat Fry. I think the sloped front gave me problems in getting the bait aligned to start it squarely. I'm sure a bait with a more blunt front end, maybe like the Ned or a worm or some other rounder bodied bait, would allow the alignment to fit better.
The two baits shown rigged would work on either head size. And On that note I did try a 1/32 ounce conversion to use this system but the head volume was not sufficient to hold the spring's leg and in fact the spring's diameter was the same size as the head of the jig, so that size was scrapped.
One thing I will say about the spring keeper. If you get the bait on to your liking it don't think Hercules could cast it off. The Fat Fry was the second one I used to get the picture and getting the first one off rendered it junk. The down side is that for baits like the 1.5" Thump-It or baits with deep ribs and a smaller core this isn't very practical.
Like this thread has shown, the interest was there and Kyle sort of steered the thought process. It was a challenge and I know now that it can be done in these two smaller sized heads. I may make up some of each size but more than likely I'll dig in the old mold box and ferret out a 1/8 - 1/4 mold that makes collarless heads and fit these springs to those heads for some walleye goodies. Like I said, a guy can't hardly imagine casting a plastic off one of the spring keepers.