Webbing with a quick release sounds good!
Clip should be a u-shaped spring thing to trap the spiral:
(That's my old one before I got fed up with it and the feeble 25mm knicker elastic and velcro strap fell apart! You can see I ripped the useless bits off!
Roughly 150 mm wide if you include some sort of pen grip thing (single or double), by about 230 mm high.
Plastic dipped - blokish black or
princess pink would be fine. Strap about 40mm wide?