"The idea is just to cut down on the cross sectional area but still have some rigidity"

This is a needlessly complicated design. If you could simply obtain some reasonably thin walled OFHC tubing
it would be a good deal easier to assemble. Can't find that? Then get some appropriate thickness OFHC
sheet, shear to length, and bend it into an L-channel of appropriate width and length equal to your
cruciform section.

It would take you a few days to make the one part. It would take an hour or less to make the simpler one.
They'll both work equally well I suspect.

A few hints: surface area counts. A lot of cooling at that temperature is via radiation - surfaces exchanging photons.

Soldered joints can work but use 95/5 tin silver alloy. Does not go superconducting so the thermal transfer is better.

Avoid complicated designs - think "design for manufacturability."

Avoid polymers, stainless.

Read Pobell. It's a good reference. There's also the cornell low temperature book.