While I'm waiting for a closed loop belt for my previous design, I whipped up a quick design to connect the drawbar hex nut to an encoder. I turned/milled the EncoderSupport in aluminum but plastic would also work. I printed the EncoderMount (hex nut to shaft) in PETG.

This idea should apply to a lot of other drawbar machines. Modification to connect this assembly to your machine is probably necessary. (This works with a Rockwell Mill.)

The aluminum is heavy enough to just insert on top of spindle housing. I get a little rocking and a flex coupling would be an improvement, but I can rigid tap M6-1.0 into aluminum with this setup which is what I needed.