Bodge recommendations sought: Attaching a top cap to a threadless headset adapter

@svenhöek Thanks – interesting ideas!

The internal adapter diameter is 20mm so too small for the FSA bung unfortunately. Drilling the bar-end cap is a good idea if I could also shave its sides, since it has a slight expansion in the body but the cap is a little too wide at the moment. And the threaded tube is a great hack – if I didn't already have a cap I wanted to use.

The next thing I'm trying is to make an DIY expansion bung from an M6 rubber cavity fixing, which I'm going to nestle inside some 13-20mm rubber washers. Hopefully introducing a bit of the bolt to that nut will expand it sufficiently to stay in the adapter. I've got some small M6 rubber washers to keep the nut away from the bolt head if necessary. Those bits should be arriving tomorrow so I'll let you know how it turns out!

Thanks again for the tips.