As Davkol said, I have the solution for that. Check my signature topics – if you use Windows then it's the PKL one (but there's more info in the xkb one). Oh – I see you're on Ubuntu so that's fine then.

My Linux/xkb files allow you to select Extend mappings independently from your layout choice (and Wide/Angle mods too, as keyboard models). It's modular, the way I like it. You'll need to set a lvl5 modifier switch (I use Caps) and the 'extend' option as described in my topic.

Eh, am I invisible? :D You just asked again about the things I just answered!

Right. :)

Nowadays I prefer providing changed xkb files which you'll find for download from my post. If you don't want to change your actual system files, you won't be able to see the changes in your GUI keyboard tools but setxkbmap can still handle them. Then you'll need to make a copy of your system xkb folder, copy/replace my changed files and run setxkbmap from your xkb folder.

I'm in the process of making some more practical scripts for this, but haven't finished it.

You can choose the Wide/Ergo mods as keyboard models if you're interested; that would not interfere with which layout you choose.

Same thing with Extend: It's set as options (-option 'misc:extend,lv5:caps_switch_lock'). So you could get the Back/Del/Arrows/etc with Caps or another modifier (there are several options for lv5 modifiers in the system already) regardless of layout choice.

Yes, I'm thinking about github. I started, but haven't gotten there. Too much real life in between my valuable hobby time! :)

Best of luck!

I edit xkb files with a text editor. NotePad++ under Windows, GEdit or whatever you prefer under Linux.

For your purposes, unless you want to change the Extend mappings you shouldn't need to edit any xkb files. If that's what you're planning, be warned that Extend mappings are a bit tricky.