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I just placed the negative directly on the scanner's glass under the transparency backlight to get these scans. I didn't care if it wasn't perfectly flat on there because I shot it through a Holga to begin with. I've heard of folks flattening it on the platen using a piece of glass, too, to varying degrees of success.
As for modifying a camera to perform this experiment, nothing beats the flexibility of a Holga. Many others have done this before me, and I more or less followed the instructions at Squarefrog. An unused kitchen sponge worked great to keep the 35mm canister in place, and I did opt for the rubber bands on the 120 takeup reel to keep the 35mm film from straying to the edges.
So I have to wonder why anyone would buy a 35mm Holga...just buy the normal one and you get the 35mm version free.
Wind a few less clicks on the film reel, and the frames overlap, giving you a panorama, Holga-style.
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I imagine this could work in many medium-format cameras, and I like the effect, so if I get my hands on one I'll be sure to try it.