Here I use a sheet of ‘fish paper’ to reduce the body tube inner diameter: MantaRay 21.86 ID, Molicell 20.53 OD (20700 format)
The material can be found here
Comes precut at 65mm and is quite economic at $1.00 for 10 feet (but they get you on the shipping). Should be some without the sticky backside in Ali.
Mine is .40mm (.016”) thick and has a natural roll. It stays within the flashlight body.
For converting the 21 to 18mm, using 2 or 3 concurrent rolls (.4mm x 3 = 1.2mm per side; close enough to 1.5 but tight fit – 2 rolls has just the right play).
Plastic jugs and the like are usually made from polypropylene. Rather stiff and too high a melting point (135 to 175 ºC) to mold with the battery therms. Also, most probably thinner. Hence more ‘rolls’.