If it’s a backpack makes sure it’s got a cross strap so you can strap it to your right seat to secure it.
A ‘messenger bag’ style make sure it has an adjustable carry strap so you can secure it over the backrest of the right seat.
Apart from that anything will work.
I’d suggest you go pay a visit to your local thrift stores as they usually have at least a couple suitable bags in the $5-$10 range.
Seriously.