In Neff's Misfit #2 thread (
link) he describes his approach of drilling and tapping (threading) a hole in the plastic and then epoxying a matching screw/bolt into the plastic. Eric offers an alternative to the epoxy for bonding to Delrin in the form of a 'super' type glue in his thread about Big fun with big Delrin (
link). The screw can extend beyond the top-body side of the plastic and then be installed in a hole or matching thread in the top body and the assembly can be secured with a nut inside the top.
For strength, choose the largest screw size you can such that the plastic at the neck or groove of the tip still covers the threads of the screw after you've turned the plastic to your desired neck diameter for the particular top. It might be a bit of a surprise to reveal the steel screw while turning and without backfilling the exposed threads with something, the string will grab in that spot and be a source of frustration.
Good Luck!