This repository contains a keymap for US keyboards that enables the following:
AltGr+A -> ä
AltGr+O -> ö
AltGr+U -> ü
AltGr+S -> š
(plus the capitalized versions)
Drop the file into /usr/share/X11/xkb/symbols (or the equivalent in your distro). Select it with setxkbmap ee_altgr.
Use the Microsoft Keyboard Layout Creator to compile the klc file or download the compiled package.
TODO: reloading w/o root could maybe be done (I have't tried this yet):
setxkbmap -device $remote_id -print|sed 's/(xkb_symbols.*)"/\1+custom([layout])"/' | xkbcomp -I[path] -i $remote_id -synch - $DISPLAY 2>/dev/null
#remote_id on muuttuja, jossa on se näppäimistön id jolle ton haluaa applyttää [ainakin toimii tolla skriptillä joka mul on; path alla on hakemisto symbols, jossa on layoutkuvauksia eli remote_id on haettu xinput list avulla thx eax255