@jwz Have you tried an ordinary rubber band inside the groove?
Or something ever so slightly more specialized, like one of these? https://www.amazon.com/dp/B0GGB2MDHT
(sorry for the Amazon link)
undefined
Thomas Sibley
@trs@metasocial.com
Piero Bosio Social Web Site Personale 
Social Forum federato con il resto del mondo. Non contano le istanze, contano le persone
undefined