I found the original at Thingiverse (https://www.thingiverse.com/thing:3036241) but the "hook" to attach the strains to was a bit to flimsy and weak for me (actually broke two times on two models while attaching the strains). So I removed the hook, widened the hole at the front and introduced a small "tunnel" for a zip tie or similiar. It fits my needs perfectly and maybe somebody else can make use of it, too.

Credits to "Kattstryparen" for the original design.