Low-cost 35mm adapter
 

My apologies if this has already been posted (a search didn't yield any hits) but this video shows how you can mate a camcorder (of one type) to an old 35mm film camera to use the latter essentially as a 35mm adapter. (Or you can view the camcorder as a "digital back" for the 35mm camera, as the video's author does.):


It's a very high-tech solution: The video's author uses a rubber band to hold the two together. And you should really replace the viewfinder's screen with a clear matte one that doesn't have focus indicators, etc. Maybe not too practical but if you happen to have a 35mm film camera hanging around not doing anything...

Re: Low-cost 35mm adapter
 

The gentleman has discovered a serendipitous ability of his electronic camera to focus upon the groundglass panel of the camera viewfinder.

P+S Technik have married up a similar mobile device to their PRO35 Adaptor as a kit to use both as a recording director's viewfinder.