You could get a regular viewfinder eyecup/setup to have the mount aspect and then build your own angle finder onto that. For a single angle (ie a fixed angle finder) its probably workable.
The image is also not reversed, it comes out just as normal to the viewer.
However what you would lack/make things more complex:
1) magnifcation - most anglefinders from the official to 3rd party (I use a Hoodman anglefinder) give you around regular sized viewing and a magnified view (normaly this only shows the image to be captured and missed off the tech details like the meter reading
2) Bulk - make the angle finder too large and you will not only have a lot of weight on the mount, but also if its too bulky you might block out the sensors above the LCD (on cameras which have an auto off feature for the LCD for when the face gets near). Clearly a problem for review and such if you can't see anything on the LCD.
3) Turning - most angle finders turn through 360 degrees (can make for interesting ways to view the image....) so that is again a tricky thing
Clearly it can be done, but complexity and time needed as well as materials and tools might make it more effort than its with in comparison to a commercial choice