1. A simple, close up diopter as the lens;
2. Some way to focus it while minimizing flare;
3. And you're in bizniz.
The simplist method would be to tape a black bag (trash bag, leftover bag from printing paper, whatever) to both the camera body and to the outer rim of the close up diopter. There's your "focusing bellows" that also helps minimize flare (see, this trick'll work even without a bellows, barrel, etc. but flare will be atrocious).
Wiggle diopter around 'til you have a sorta recognizable image. Presto.
The sorta sharp center, very soft edges is due to heavy spherical aberration.
Attaching the diopter to a bellows or barrel, something with a controllable sliding or helical device, will allow for better control of the entire rig.
However the black bag, or a bellows without a rail, a detachable rail, etc., will allow for "movements" and funkier effects using the selective focus technique.
Besides my ready-made Spiratone Portragon, I've experimented with a Vivitar 10x close up diopter, an old Wollensack enlarging lens and a cheap loupe. I tend to come back to the Portragon for convenience.