3D printed spacers (2x) (like the leftmost on the attached image)..
I made that cell to mount round prism next to diffraction grating.
Another way is to cut spacers from machined aluminium tube with mitre circular saw..
like this one, only bigger. I am using 180 mm saw from Bunnings (it was $50 when I bought it).
And, you can try with small spacers placed at 180° inside the filter holder