I thought I'd put this up here for the giggle factor as much as the useful factor. This is a Yashica-Mat where I removed the top part of the finder hood, then installed a Hasselblad NC-2 prism (all of $20 on ebay).
The base plate of the prism unscrews, and then the prism fits into the opening almost perfectly. A spacer was needed on each side of the screen to raise the prism to a height that gave focus on the screen. This turned out to be about 5mm wide by 6mm high. I used wood and 3m transfer tape to hold the spacers in place and to grab the bottom of the prism. Electrical tape was then wrapped around, as you can see.
I wouldn't pick the camera up by the prism or hold it with this tape attachment. If I keep it, I will rebuild the attachment to make it secure.
It actually works quite well. Makes a 'chin level' 'SLR' feel camera without the mirror slap (and with the obvious parallax error). Focus is quick and precise, although it also brings every scratch and dust speck on the screen into sharp relief.
The biggest downside is that the prism sticks out behind the back of the camera and bumps my chest when walking if I carry it around my neck.
This was actually inspired by someone who had a Rolleiflex on my local craigslist for months with a Hasselblad chimney finder in place of the typical waist level finder. I used this Yashica-Mat because it is my trashiest TLR; I might do a final version on a Minolta Autocord.
Another view- http://farm5.static.flickr.com/4019/...c9d76a32_b.jpg