I know this isn't the proper way, but I used one (1.5 gallons, I think) as a Q tank recently for 2 small serpae tetra I bought.
I added gravel and a peice of fake plant for cover from my established tank and set it on a reptile tank heater pad to keep the water warm.
Daily water changes kept everything in check and they were only in there for 2 weeks.
It worked good in a pinch.
Edit: I forgot, when my husband brought home my
betta girl, I set this bowl up the same way until I was sure the cycle was completed in her 5g, about 5 days to a week.