How to map the Caps Lock key to Left Alt in Ubuntu +18?

Question

I've been looking for a way to solve the aforementioned problem.

I tried

xmodmap -e "keycode 66 = Alt_L"

but apparently the result is that the CapsLock key gets mapped to Left-SHIFT, which is not the intended behavior.
From my searches I understood that xmodmap is obsolete and I should use setxkbmap instead. However, I found it very hard to remap a key that is not part of the default options of xkb (e.g. I could map CapsLock to Left-CTRL but not to Left-ALT).

How can I use setxkbmap to achieve this goal?

Answer 1

I started considering the solutions from here and here, but I couldn't make them work.

I found the solution here:

https://unix.stackexchange.com/questions/9635/how-to-assign-another-modifier-to-alt-key-for-x11

It achieves what I was looking for, although in a rather ugly way (I would very much prefer to create a new xkb option with this remap, but I couldn't make that work)

---

Update

The solution above doesn't allow me to keep two different keys for LALT. Instead, I followed this guideline with the following symbol definition:

// This changes the <CAPS> key to become an Alt modifier,
// but it will still produce the Caps_Lock keysym.
hidden partial modifier_keys
xkb_symbols "alt" {
    replace key <CAPS> {
        type[Group1] = "ONE_LEVEL",
        symbols[Group1] = [ Caps_Lock ],
        actions[Group1] = [ SetMods(modifiers=Mod1) ]
    };
    modifier_map Mod1 { <CAPS> };
};