I finally made a digital TLR. The camera is a baby 4x4 Primo Jr which has a 60/2.8 lens. So it becomes a 90mm standard lens for 36x36mm digital back.
This is a prototype that is able to take pictures and prove the concept.
There are still quite a few issues: The shutter speed is not accurate (old camera), the infinite focus is not achieved, and therefore the focusing is not accurate. Also, the back frame (Kiev 88) is not permanently glued to the body yet, so the light sealing is not tight.
But it is a fun project. And hope that it gives some idea to the DIY/MF community so that someone may eventually come up with something better and more professionally made.