1. Masking Plates

You may have a floppy disk with a black plastic shutter.. That's it for the parts of masking plates. Some floppy disks have metallic shutter and they are not suitable for masking plates because reflection of light gives many flairs to the film.
Disassemble a shutter plate from a floppy disk. It's a U shaped very thin and smooth plastic plate. Cut the plate from "U" shape to two "I" shape plates, and carefully cut a plate to appropriate size for the masking plates. We'd better cut a plate slightly larger than the size we need, and then adjust its size by sanding down. We need two masking plates exactly the same height with the dimension between the two film guide rails of the base camera, or the camera will have slits on top and/or bottom of masking plate(s). These slits, if the camera has, will make the image not square but square with some strings.
 

Black plastic shutter of a floppy disk is good for masking plates (the part indicated by the white arrow.). Metallic shutter is not suitable for this purpose.

Sanding down a plate to the size what we exactly need (left). And mark the position of masking plate precisely. (right)

Attach the masking plates behind the shutter with very thin double-face adhesive tape and check whether the masking plates don't come above the height of film guide rails. If they do, we need to detach the plate(s) and sanding down to thickness less than the height of the film guide rails. If a masking plate comes above the film guide rails, not only the plate makes scratches to the film, but also the film plane will be off the focal plane..
 

2. Finder Screen

I put two narrow tapes onto the finder screen of EOS 650 to indicate image area of 24mm x 24mm format. If your camera has spot and/or center partial metering, you can mask the area other than 24mm x 24mm image. It should look much smarter than what I did.  Unfortunately EOS650's partial metering works only when I push AE lock button with unwilling AE lock function. It's not useful and therefore I did not mask the finder screen but add two lines with narrow tapes to use evaluating metering system.
For this modification purpose, we'd better pick a camera with finder screen exchangeablity up for the base.
 

Finder screen with a pair of narrow tapes. This indicates the area of 24mm x 24mm image.  If your camera has spot or partial metering, then you can mask whole the area other than the square image and get neat finder veiw. But EOS650 does not have useful partial metering and therefore I put lines instead of full masking, to enable myself to use evaluating metering.

3. Test

The test result was great. I could enjoy compositions quite differently from those with 24mm x 36mm camera. I could also enjoy 500mm photography and macro photography with square format. This camera offers 24mm x 24mm images in the same frame pitch as 24mm x 36mm, therefore I can handle the negs as if they are of 24mm x 36mm (actually they are 24mm x 36mm images with 6mm black strips on both sides). Mounting a slide to a plastic mount, printing, scanning….everything is compatible with 24mm x 36mm negs. 
I've had a floppy disk, narrow tape and sandpaper, and just bought double-face adhesive tape at A Dollar Shop. So total cost for the modification is $1.

This methodology should be applicable not only to EOS but also to any kind of 35mm SLR cameras. Would you try one?
 

View of a film taken by the EOS650 Square. Frame pitch is unchanged from that of 24mm x 36mm, so everything is compatible with 24mm x 36mm negs. It's very useful.

	It was not taken by Hasselblad but EOS. Don't you think it's funny? EOS650 Square, Sigma AF70-200mm F2.8 EX HSM, Kodak E100VS
	EOS650 Square, Sigma AF70-200mm F2.8 EX HSM, Kodak E100VS