I didnt take enough pictures when I fitted mine in April and I cant get enough of a view today without taking the spoiler off.
From memory the nut supplied with the twin arm washer moulding goes on top of the wiper arm and then the 3d printed bit goes on top of that without cutting anything off as the inside is shaped to fit the "nipple" and the grub screw tightens it onto the nut.

The eBay instructions are useless.

I dont recall cutting anything to install it as I'd just the weekend before stripped the wiper assembly and rebuilt it with new rotary seals at the inlet to stop it "raining" inside the boot every time the rear wash wipe wash used.

Image below is a close up of it fitted. Maybe try putting the supplied nut on top the wiper arm and test fit the 3d printed part over that?

Sorry cant be of more help at the mo.

Click image to enlarge

I’ve pretty much tried every possible option. Agreed that the eBay instructions could be better written.

Just thought it worth flagging before anyone else potentially destroys their wiper motor.

I did message the seller a month or so ago with all my feedback and requesting clarification of the nipple comment.

A potentially great idea with poor guidance.

Incidentally, I don’t think the 3D nut I’d fit for purpose as the threads don’t appear to have sufficient strength over the long term to retain the wiper arm on the spindle and you can’t tighten the nut with sufficient torque to clamp down the wiper arm ring.