This evening I finally got it together....but made a mistake. More on that later.
I chucked my nylon plastic part into a vise, and marked four corners on the face. Then took a small, square needle file and made some grooves on each one, working up inside the bore.
Next, I held the shaft with a vise, set the plastic end on top and whacked it with a hammer. After a few times, I managed to fully seat the shaft into it. I can see where the metal corners of the shaft 'forced' the plastic to yield as I drove it home, creating four lines 90° apart. Didn't look like it would ever give.
I turned to the motor shaft end. I'd underbored it to be safe, then searched for a 9/32" drill with no joy. The OD of the shaft I'd previously measured at 0.3". A 9/32 is 0.28". 17/64 is 0.27. Unfortunately, I had neither.
Then I found a pen tool reamer that was very, very close to 9/32 so I thought "Why not"? It wasn't exactly straight though and was enough to widen it a fraction over what was necessary.
I tried it out with the string trimmer attachment and it worked for about 15sec., then I could hear the motor side of the coupler slipping. Sure enough, it was loose. @#$%!
I decided to cut a piece off of some 3/4" AL plate. It sure was slow going with a hack-saw. Then I spied a piece of 1" square Corian and thought of trying that. (I'm out of 1" round Delrin). However, the 3-jaw Taig chuck wouldn't hold the square Corian and I don't own a 4-jaw chuck.
So I marked and drilled the center to slide it onto my pen mill to turn it down to 3/4" round. The Taig was cutting very well and I had fine whisps of Corian flying right off of the tool.
Only then did I think to check if the drilled center was too large for the square end.....yep! Screwed the pooch on that one.
So I've decided to hang it up until tomorrow when I can get to the local HW store that carries drills in No. sizes. Even if the bore wasn't too large for the square end, I still wouldn't have an appropriate sized drill for the spline-end. And the dia. is too small for my inside boring tool. A more experienced machinist could figure this out, but I'm just a beginner.
It did occur to me that I could file a flat on the motor shaft, and drill my coupler for a set screw or two. Not sure so I've vetoed that for now.
I'll also have to deal with the assembly of order if I'm to press-fit both the flex shaft AND the motor shaft to the coupler. One is very easy because I can tap the end of the coupler, the other though requires me to set the motor upright on the bench, position the coupler and upper drive-shaft in place, then tap the end of the flex shaft to seat it into the coupler.