Attach Spring to a rigid element

I am looking for a way to save CPU time since my model got quite complex. Is there a way to tell radiOSS to calculate one or several parts as rigid?

I tried using the checkmark rbody for the relevant part but it caused a few errors.

A simplified model is attached. I'd like the upper one of the cuboids to behave like a rigid.

There seems to be a kind of hourglassing? happening as well, which I didn't manage to solve.

Advice on any part of this is appreciated!