I converted some led torches to rear lights.
I had a broken lorry tail light in my workshop so I cut out some round discs of red prismatic lense and glued it to the business end of the torch with Araldite.
Works fine.
I have one permanently fitted on my Brompton, and my Hungarian folder, and my Ratrike. Arch also has a couple.
Here is the one on the Ratrike, it is set up as a brake light.
It is a waste of time to convert them really, as a perfectly acceptable rear light is so cheap to buy, but it was good fun making them.
Also I only use them in conjunction with a legal rear light anyway.