Not sure if this is of interest to others - but we recently made some simple, removable cup holders that attach by Velcro to the box weave carpet (see photos). The Librands door mounted versions are both staggering expensive and for me in the wrong position; these cost less than £6 each.
The foldable holders themselves came from eBay and we then suck a 50mm strip of Velcro on their back with contact adhesive, secured with three aluminium rivets. Seems to work quite well, but not yet tested under extreme braking!
Have on my PlusFour the side pockets supplied from the factory. Was not quite cheap and also not really useful because actually nothing except a few papers nothing fits in. I bought a net for 10 euros on Amazon and attached it to the side pockets with the existing screws. This can now be used as a cup holder.
Had today out of frustration because of my scratch tinkering day 😎