This repository contains a keymap for US keyboards that enables the following:
AltGr+A -> ä
AltGr+O -> ö
AltGr+U -> ü
AltGr+] -> õ
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.