Yep, plain old folding chair, with no arms; not a camping chair, but the kind you put out at a church social picnic.
Get one that sits the highest and tallest; they're not all the same height. The taller, the easier to play without hunkering down in an uncomfortable position.
I've also added some extra height to my chair by sticking three-inch iron pipes up each leg, held in with duct tape. You can add all the height you want. My banjo playing buddies love this tall chair.
Duct tape and bluegrass music...what else do you need in life?