I've used a pinhole body cap on an M42 SLR on several occasions. At 43 mm projection distance (IIRC, that's flange to film for M42), with an optimal size pinhole it's possible to sort-of hand hold with film pushed to EI 1600 or higher.
If you have an existing plate camera, field camera, press camera, or other large format camera, you can unscrew the lens elements from the shutter, make up a pinhole that slips into the front of the shutter, collapse the bellows (most of the way) and get good results without spending a dime -- only an hour or so to make the pinhole insert -- and you'll already be set up to develop the film in that size. And if you don't, you can widen your 645 some if you look for a pinhole lens cap with a cone that juts into the body, and use it with mirror lock up. This will let you roughly halve your projection distance -- which, if you reduce the hole size enough to remain optimal, will significantly improve your overall sharpness.
