Decided to try the super easy thing first. Magnets.

They hold the assembly to the van, but not a good enough sideways locator, nor , I think strong enough to hold overall.

I did try screwing in a couple of the stock screws, backwards, facing out, as locators. But that made placing the light assembly onto the van too fiddly.

Private blog post showing pics of magnets etc here
http://shufti.blog/2020/02/23/private-magnets-on-tail-light/