I have the mold and use mine primarily for walleye, but the diameter of the head end of the bait should snug up to a ned head nicely. The end of the grub is slightly rounded so I'd simply take a slice half way thru the end segment to offer a flat end for the jig head.
The 3 1/2" Thump-It is very much under-sung in my opinion. I've had days when the 3 1/2" Thump-It really hammers walleyes while the Rippers go un-touched.