After some missed tests, I made this prototype. It will be enhanced when I find better materials (for example, I used cardboard and I think aluminium would be better). Until now this works fine (I made it 2 months ago) so I think it will not essentially change... but any ideas are welcome!

This is the holder and the WC box about to be fixed...

...and in ther final position.

Here is the Itty Bitty WC box 1.0 and the new 2.0.
I loaded 1.0 with very basic colors: Winsor yellow, alizarine crimson, Winsor blue, Hooker green, burnt sienna and sepia. On the last weeks I felt the need of a palette just for values. I made 2.0 with an ice mint box and loaded it with "non-color" colors: raw umber, burnt umber, Davy's grey, Payne's grey, sepia and ivory black.

There's a new hi tech version here.