The process was actually very simple, I simply turned a slightly tapered reformer out of an appropriately sized PVC coupling. The taper is about 1mm over the 50mm length with the target diameter coming at about 20mm. I was actually surprised how nicely the PVC turned and all three of the ones I bought to play with are 100% functional.
The ultimate finish may not be 100% but the function is there. In the above referenced coil I was able to recenter it and move it without any rub. The real test sample was a 5" basket that took a rather hard hit to the coil and was impossible to center. It was only mildly eccentric and had a distinct kink where the slit in the former was. 30 seconds in Dave's Reform School for Lowthers and another 30 seconds on a magnet had it centered and passing signals without rubbing.
The lathe is still set up and it only takes a few minutes to turn these so if any of you out there use lowthers and find yourselves in need of such a device, chime in and I'll send you one.