I'd make it a small 5' cloud of fumes that you can move as an action on your turn.
''Until the spell ends, the magic bonfire fills a 5-foot cube. Any creature in the cloud's space when you cast the spell must succeed on a Constitution saving throw or take 1d10 poison damage. A creature must also make the saving throw when it moves into the cloud's space for the first time on a turn or ends its turn there.
You can use your action to control the cloud. You can move the hand up to 10 feet each time you use it and it can pass through small holes, narrow openings, and even mere cracks, though it treats liquids as though they were solid surfaces.
The cloud vanishes if it is ever more than 30 feet away from you or if you cast this spell again."