Patty,
If you have a local Tractor Supply Company, you may be able to use one of these feed troughs. I've often thought they would work well for bananas, since the roots spread more than they go deep. It's an oval container made out of very thick/rubbery plastic. I have a 46 gal continer that is more deep than wide that I currently have a fig tree in.
Here is a link:
Tractor Supply Company - Stock Tank 110 ga