This is a Yashica/Contax right angle viewfinder, which can be obtained from ebay for under $20:
I decided to 3D-print an adapter allowing this finder to be attached to a Takahashi polar scope, so when using the polar scope an observer can avoid a stiff neck. The adapter can be found here.
Here is the Contax right-angle viewfinder attached to the polar scope on a Takahashi Space Boy mount. The adapter has two holes that need to be tapped for M4 for the set screws. 3D printing technology isn't repeatable enough at a sub-0.5mm scale to allow a tight press-fit on both the Contax finder and the Takahashi polar scope, at least when using PLA (succeeding prints may be too loose or too tight, hence the need for the set screws).
Potentially if using ABS or HIPS it might be possible to exploit the material's flexibility for a press-fit on both the Contax finder and the polar scope eyepiece end.