Custom X KeyBoard layout for English (UK) + Catalan, which gives easy access to:
ccedilla
(ç) via AltGr+c.ntilde
(ñ) via AltGr+n.
Of course, the Shift modifier will allow typing capital Ç and Ñ.
Bellow you can see the keyboard layout:
Simple, you just run the script from the root of the source folder:
./INSTALL
Apparently, in Debian/Ubuntu systems, you also need to run the following after the installation:
sudo dpkg-reconfigure xkb-data