Your best bet, as far as I’m concerned, is the Kodak Duaflex. One of the models (I think the earliest version) doesn’t have a cover for the viewport, which makes things slightly easier to rig up.
Another idea along the same lines, courtesy of a fellow photographer, is shooting through a old oil lamp chimney like the one to the left (stick the lens through the base.) You may have to mask it off a little, but you get interesting distorted vignetting with a clear center image.
Edit: I’ll post an image using an oil lamp as soon as possible.
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