Configure some less-used key (right win) as “compose key”. For Ubuntu/Gnome, this is in System->Preferences->Keyboard->Options. Typing RWin+a modifier then the letter will result in:

rw+’: a-á e-é i-í o-ó r-ŕ s-ś z-ź w-ẃ

rw+`: a-à e-è o-ò

rw+<: z-ž c-č

rw+key pressed twice: a-å o-° e-ə

.. and a lot of others, the full list of shortcuts available depends on the GTK implementations and can be checked in gtkimcontextsimpleseqs.h