Graham ,
You could certainly rig up a compound slide but its probably not nescessary for this one task . If you can get lathe to turn reasonably slowly you can true small errors in the rings with a file using a block of wood as a support and guide .
I've done this sort of thing using a power drill in the vice as the only 'lathe' available and succeeded in making adequate small steam engine parts .
Before even that I and a friend of mine made simple model railway parts using a hand drill and needle files
I never came to any harm but please take great care if you are not used to using power tools .
Actually given the rough and ready nature of the proposed construction you could true up the rings just by hand filing – no lathe needed at all .
Regards ,
Michael Williams .