If it helps, I measured mine and it looks like you need a collar with an inner diameter that will accommodate an 88mm lens.
I am sure that part is long discontinued by Canon so your best bet would be ebay and used camera stores looking for the used part. This was the only lens in that series with a removable tripod collar and the collar has cutouts to fit over the lens mount screws so the original Canon has to be oriented by the colored dots to mount/remove the collar.
It is a wonderfully sharp lens design!
And on edit: If you can't find anything else, google telescope rings and you might find something workable.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
