I also had a go at modifying some files to fit. I splashed out on some £2.50 wilko files, chopped them up, pressed them into an aluminium bush and filled any gaps with epoxy. When the epoxy was set I chucked the file in the lathe (with brass shims over the jaws) and trued the bush to the file. This is how they turned out.

If this method works ok I will get some more files and do the same thing. As a by product you also get some stubby files with no taper that may be useful???

I also bored the coupling and made the reciprocating shaft.
This is a mock up of the bits so far on the lathe (clearly I am missing some table supports... before anybody points that out

). I will probably connect the drive shaft to a 19mm collet in the collet chuck rather than the 3 jaw as shown. The picture is a bit confusing as there is a lathe and to tool holders cluttering the background, but hopefully you get the idea...

next jobs are to mount the other half of the jaw coupling to a shaft, make some table supports, make the delrin puck and tidy it all up with a splash of paint.