Lots of great cage manufactures out there, I've used Boaphile, Freedom Breeder and slant front Neodesha in the past and currently use ProLine, Animal Plastics and Barrs cages.
Had some slight sagging in Boaphile and old style Prolines but that was caused by placing heavy things on top of the stack (like large pieces of grapevine) and not from the cages themselves.
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