It looks like the end cap sits in the hub and is secured by a spacer which is a very tight fit, but I cannot get the spacer to press into the hub. Am I doing it right?

Are they different to the Sun Ringle Chargers? These just have a space for each side, but the one on the disc side is slightly longer as the bearing sits deeper in the hub.

They were just push fit, with o-rings holding them in iirc.

The 15mm spacers are like that, the 20mm end caps are a different design. I ended up using a stem as a drift to force the spacers into place, not ideal but I needed them for Hit the North this morning. Will call Hotlines on Monday.

Ah, the 20mm ones on my Chargers are the same, bar the diameter, to the 15mm ones