So I found these: https://www.koni-na.com/en-US/NorthAmerica/Products/Accessories/Race-Single/
I contacted Koni about them and got this response.
I proceeded to ask if there is a cut a strut option for the Reatta (which I doubt), and if not if they would be willing to add it. I also asked if he could provide me with a list of the cut a struts that they do offer. I have not heard back yet.
That said, I think the 8610 Race cartridges could work. They only have a 6" stroke, but other than that, I think would fit pretty closely. I was thinking the same thing of just fabricating a custom housing for the 8610 cartridges.
Still hoping to hear back from the Koni Rep. Maybe we can all bombard his email and he'll think it is a big business opportunity for them to add a cut a strut for the reatta . I would think this would be the perfect kind of application for a cut a strut type solution.
UPDATE:
So I'm dumb, that email above was actually his reply to my question if they had a cut-a-strut for the Reatta. In his original email he pretty much said the same thing (hence my mistake) except he did mention that the 8610 Race cartridge is held in by a gland nut from the top (compared to the Cut a Strut being a bolt through the bottom of the housing).