Looking for a storage container

Those plastic storage containers with the attached flip top lids would probably be the best type. From what I've seen, they come in 27" x 17" - which would fit your width, but not your length; and 21.5" x 15.5" - you could fit two of those side by side with some room in between maybe.

I'd say your best bet if you want as much length as possible is to build something to fit. I doubt you'll find many containers 48" long but only 18" wide - especially when you'll also need something with a lid that won't come off while you're driving down the road.

I was thinking that, one sheet of material would do it