I've stayed in a few hotels that don't have anywhere to put your showergel except on the floor, then as you bend down to get it, you get water blasted in your ear. :(

This may not be the prettiest solution, but it does meet all my objectives. I wanted something that would be really easy to print, and that would travel well.

Printing couldn't be easier, absolutely no supports required at all! And for travel, the whole thing collapses down in to its own tidy little case. The lid is a little loose (didn't want it getting stuck), so should be secured with a small elastic band. Total weight is about 60g, so won't make much dent in your luggage allowance.

The hooks are designed for any thickness of shower door up to about 10mm. Alternatively, you can use suction hooks (sold separately), the type used for sticking Christmas lights to windows work perfectly and are really cheap and sold everywhere.

The usable shelf area is about 51mm x 109mm. It's designed for travel-sized bottles, but will take a couple of large bottles. The whole thing should scale up fine as long as you scale everything equally. I wouldn't go smaller though, as the plugs wouldn't be strong enough.

All feedback and suggestions welcome.

--- UPDATE: 29 AUG 2020 ---
Just added a pair of "J-Hooks", which hang from the mesh of the shelf. With a bit of careful arrangement (see photo), you can still fit the whole lot in the case! Also, to stop the bits rattling, I use a thin sheet of foam cut to size, then keep the whole lot together with an elastic band, or hair band if you prefer.