If you want something durable but not that expensive have you considered Lojel? It's a Japanese brand that doesn't seem that popular in western world. My parents have an old Lojel thay have had for maybe 20 years? It's been checked in and beat up a ton, it's scratched but it never failed. Perhaps anecdotal evidence.
But anyway they're priced competitively and have a few models that spin and use latches rather than zippers (more durability) they even have a model that has an aluminum frame inside.
http://www.lojel.com
Horizon
Octa
Groove (frame version, aluminum frame)
Kozmos
Are all models that are zipperless. They start at about $200-400 depending on size and material you want.
The ones made of polycarbonate use Bayer branded PC. Should be solid.