I inspired myself from this design : https://www.thingiverse.com/thing:5223379
Not a remix tho as I went from scratch. I wanted something a bit more realistical, as the real stuff is already quite symetrical.
Use the test_mirors_jwst to check the match with your mirors. It must be a nice fit, not snug but not to much tolerance. The original uses 87.2mm pockets for the mirors, scale according to yours.
I did cut the part to fit my cr10v3, the full model is in the file too.
Designed with Fusion360, cut with Meshmixer
The author hasn't provided the model origin yet.