Looks like a good tip - also bought a more expensive one (Hawk peng)I had the same problem with a Navitar 25mm f/0.95. I ended up searching through eBay and buying 3 different C-mount adapters labeled "slim" or something like that. Three weeks later I found that one of them, indeed, was slim enough to allow infinity focus.
No, unfortunately I don't remember what vendor. But I've also found that the same item bought from the same vendor several months apart may not be identical, so I think a lot of mix-and-matching goes on behind the scenes with the Asian suppliers.
I also remember they cost between $8 and $12 each, so the crap-shoot wasn't that expensive. And I could use the 'fatties' on my other C-mount lenses, which weren't so demanding.