You can call ap and tell them you need exactly 48x24x12 on the inside and they would do it but you would be looking at waiting longer for the cage. They should list both interior and exterior dimensions but unfortunately I don't know of any companies that do. Your exactly right about having it moved closer to center would give proper gradient, not all the way in the middle obviously but either right center or left center depending on which side you want to mount it on.
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