That Kaiser one I linked to is 10.5mm according to the internet. I'm sure I had a cheaper one that was more like 6mm. But as you say, not exactly accurate.
Look for line levels. We used to use them when drawing cross sections in archaeological features. Hang of a taut string to make sure the string is level to give you something to measure from. They're usually pretty compact.
That Kaiser one I linked to is 10.5mm according to the internet. I'm sure I had a cheaper one that was more like 6mm. But as you say, not exactly accurate.