having used a couple a couple the upright part does need some 'give' in it, otherwise your not that much better off than if you just used a clip or clamp.
My advice would be to buy a couple of long strong springs and use them for the main part of the 'stem' and then have a small piece of the end to hold the cable and the springs together.
IMO the best bungee on the market is this;
So if I was making a custom one I would try and replicate it as much as possible
Does a mouse bungee even do anything? I have always been curious but have never really felt the $30 justified.
Yes, never having your mouse snag or have to deal with cables etc for your mouse is really super great when gaming and allows you to always have a smooth experience when using your mouse. The bungee I posted I think is less than $20 and is worth every cent.