Campbell very kindly cut my nylon stock into something that will hopefully hold the motor centred on the input shaft of the gearbox. You can see the outer diameter has been cut to match the motor and the inner diameter is being bored to match the sleeve around the input shaft. We aimed to cut the two diameters concentrically and produce a close fit to the motor and the gearbox.
To do this we faced the same problem that Precision Engineering complained about when cutting the input shaft -- the part must be taken out of the machine to check the fit. Unlike the professionals, we don't have much hope of accurately re-centering the part. Nylon is a little flexible so we aimed for an interference fit rather than a slip fit. This means that we are less likely to over machine and make the coupler loose.
In the end we made it much too tight and had to cut again. We managed to re-center it fairly well and make another cut deep enough that (hopefully) the part isn't too oval. The center hole is still a little too tight for easy assembly, I may have to use some sandpaper.
I have to slice a little off the outside diameter to clear a projection in the bell housing and then I'll be in a position to remount everything in the subframe.