setxkbmap -model pc105 -layout "gb" -variant "extd" -rules ~/xkb/rules/base

<variant>
  <configItem>
    <name>custom</name>
    <description>English (UK, custom)</description>
  </configItem>
</variant>

$ setxkbmap -query -verbose 10
...
Trying to load rules file /usr/share/X11/xkb/rules/evdev...
Success.
Applied rules from evdev:
rules:      evdev
model:      pc105
layout:     us
variant:    altgr-intl
options:    caps:hyper,compose:menu
Trying to build keymap using the following components:
keycodes:   evdev+aliases(qwerty)
types:      complete
compat:     complete
symbols:    pc+us(altgr-intl)+inet(evdev)+capslock(hyper)+compose(menu)
geometry:   pc(pc105)

/usr/share/X11/xkb/
├── compat       # ??? dark magic here, avoid
├── geometry     # as in physical, eg for generating layout maps
├── keycodes     # helpful for translating keycodes (from xev) to <FOO>
├── rules        # "evdev" is the important one; *.lst & *.xml are descriptions
├── symbols      # main layouts, variants, optional overrides
└── types        # ??? dark magic here, avoid

$HOME/.config/xkb/
...
├── rules
│   ├── evdev-local
│   ├── evdev-local.lst
│   └── evdev-local.xml
├── symbols
│   ├── my-fun-capslock-options
│   ├── my-US-Dvorak-layout
│   └── my-ZWERTY-layout
...

setxkbmap -I $HOME/.config/xkb
  -rules evdev-local
  -layout my-ZWERTY-layout
  -option myZWERTY:option1,compose:menu,fun:caps_is_insert