Thanks for the advice,
I've ordered some chinese ballscrews on ebay and when they get here I'll figure a way to get them fitted. Closer to the time I'll cast around on this group for somebody near me with a mill. I tried looking for dimensioned drawings of the 1204 ball nuts but found 2 different sets of dimensions, so I guess it'll be better to have them in hand and measure up before going any further.

In the meantime I'm going to go ahead anyway and try making some delrin acme nuts, just to see how it works (I just enjoy trying new things). Whether they work or not won't really bother me since I've now committed to ball screws anyway.