I built a video tap for my Arri BL years ago. I found a little camera and lens and then had a machinist help me with building the hardware. It wasn't too difficult.
Does your camera already have some kind of video tap ready hardware installed? I mean the split mirror box with mirror? If so, it should not be so hard. Just remember when you design your camera mount to include adjustability to align the camera to frame the ground glass correctly. You will also need to be able to adjust the focus and iris of the camera lens in some way, and perhaps the camera-ground glass distance to get the size correct.
These days you may be able to build the thing using a 3d printer, which would make it much easier.