I couldn't mix mappings from two models (rx-51 and ukeyboard) so I created my own one.

Unfortunately, there's some problem in the mappings. As follows:
Do you have any idea what's wrong with the mapping? You can take a look at it here: https://github.com/marmistrz/marmistrz-keyboard/blob/master/xkb/symbols/nokia_vndr/marmistrz

If you want to try the mappings, you need to install the deb package. It's needed for xkb to detect the new layouts. (it modifies /usr/share/X11/xkb/rules/base)

The package in the repos puts the file in a wrong directory, so you need to put the file on your own.