I have had occasion to customize some of my DC-DC
eval boards for in-vacuum testing. What I did was to
make an aluminum shim that fit the opposite side of
the board from the DUT (surface mount, many thermal
vias) slightly oversized, and relieved where there were
through-hole stubs to respect. Some thin Kapton film
made a decent insulator without too much thermal
impedance, between the board and the slug. I used
the board corner holes and equal height standoffs to
the aluminum slug, mounting to a larger aluminum
plate.
You don't really need a lot of force, just positive
interference. If I were not needing to swap samples
I would have used some of that "arctic" silver filled
epoxy between board and Kapton, Kapton and slug,
slug and plate, and make it all a solid stackup.
I might recommend tapping the slug and using
it, plus corner standoffs, to mount the board to the
case wall.