I think what people are referring to with the inner tubes is to cut a cross section out of one, not using the length of the tube.

I'm unimpressed with the elasticity of the inner tube bands I've made, I'm going for some industrial strength actual rubber bands from McMaster-Carr. 1/2" in width and available in different lengths in either natural rubber or red. Cheap enough to buy a box and stick into my preps box.

Other possibilities, get some heavy duty elastic banding from a fabric shop and stitch it into loops. Use a decent box type stitch and it'll stretch more and last longer than most rubber bands. Plus it comes in "tacticool stealth black"