Lots of options. I just use a notched wooden stick, marked in 1g increments. Not terribly precise, but functional. I wouldn't use plastic in boiling liquids. Another, and more reliable option is a sight class, or marking your kettle. Or, measure volume as you sparge. Have a 1qt pyrex measuring glass? I've done that before...kind of annoying, but very precise.
Also remember to account for expansion. ~4% at boiling vs 60°