I didn't really plan on selling these, but I'm glad people are interested.
The .STL file (a kind of 3D model which is used in 3D printing) was and is attached to the original post for those with access to a printer.
Like I say in the first post, there are a few revisions I would like to make to the clips which hold the adapters into the Rota wheels. The ones that are on my car (and in the attached file) work. They have been on my car for over a month and about 800 miles of somewhat spirited driving. However I just eyeballed the fitment and they could engage a lot better. I used a dab of silicone on my car to keep them really secure, and I'd like to make them fit well enough to make that unnecessary.
I made these using OpenSCAD, which is a programming language for making CAD models. I'm a programmer, so that was the most intuitive way for me. Obviously you could make something similar in SolidWorks, AutoCAD, CATIA, or whatever CAD software you're familiar with.
The material is VERY cheap. These weigh probably ten grams each, and a kilogram of ABS plastic filament is around $25. That's less than $0.10 an adapter. The time is the most expensive part as my printer is very crappy and slow and takes about 40 minutes per adapter, but even still they're not exactly pricy.
Since there's interest I'll make some revisions to the model and post it. I'm not sure I'll have the time to make and sell them myself, but you can use a third party like Shapeways to print the model out for you.