Tom,
"I might as well get some use on my new mini lathe. I'm thinking of delrin or brass."

For your adapter use brass or aluminium, Threads in delrin are harder to do than expected unless everything is set perfectly.

Are you experienced with lathe use & threading? if so read no more. Otherwise there is a simple way to remove and return your piece (e.g. to test fit) without losing thread registration.  This assumes there is a flat face or ring which registers against the face of the chuck or (less good) chuck jaws. Having progressed your thread to the point you wish to test it, back off/withdraw the cutting tool but leave everything engaged; then use a felt pen and make a line on the side of the workpiece onto a chuck jaw or face of the chuck. Now you can take it out of the chuck, play with it, test, then return it to the chuck in the same position and make further cuts as required. Accuracy is improved by tightening the chuck using each key position sequentially, and the same when you rechuck it. Don't rub the pen mark off!

You should be able to place the piece back in the chuck to the same depth and square (flat surface register) and the felt pen mark will let you have the thread back in alignment with minimal change. Make some test passes with the cutting tool backed out and progressively feed in until cutting is resumed.

It is easier to do this than just hope your part will fit ( if your metrology is first class, then no worries).

Not a machining forum so we shouldn't expand this too much.

Cheers,

Len.