Custom Keyboard Layout

Download or create a custom keyboard layout. See /usr/share/X11/xkb/symbols for examples.
Create ~/.config/xkb/symbols and ~/.config/xkb/rules.
Move your layout file without extension to /symbols.
in /symbols, create an evdev file with the following content:

! option   = symbols
layout_name:basic  = +layout_name(basic)

! include %S/evdev

Create an evdev.xml file with the following content:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xkbConfigRegistry SYSTEM "xkb.dtd">
<xkbConfigRegistry version="1.1">
  <layoutList>
    <layout>
      <configItem>
        <name>layout_name</name>
        <shortDescription>2-letter description</shortDescription>
        <description>Layout Name</description>
        <countryList>
          <iso3166Id>2-letter Language code (UK) etc.</iso3166Id>
        </countryList>
        <languageList>
          <iso639Id>3-letter language code (eng) </iso639Id>
        </languageList>
      </configItem>
    </layout>
  </layoutList>
</xkbConfigRegistry>