Cut a quadrant out of the spacer so you can drop it in place once the axle is in, before locking. Still going to be a fiddle though.
How is the hub itself finished? Is there an insert in the end that you could remove, and then get a machine shop/model engineer/tool maker/friend with a lathe to copy but make 1.5 or 2 mm longer?
Couple of ideas:
Cut a quadrant out of the spacer so you can drop it in place once the axle is in, before locking. Still going to be a fiddle though.
How is the hub itself finished? Is there an insert in the end that you could remove, and then get a machine shop/model engineer/tool maker/friend with a lathe to copy but make 1.5 or 2 mm longer?