If you get serious about this, I have a contact (friend of a friend) that will build you a custom box. He is in Richfield Utah so not crazy far from where you are. PM me if you want me to dig up his contact info.
Larry, Stephanie Park City, Utah
2017 31SL King, 8K axles, Disc Brakes,17.5 Goodyear H tires,BU Camera,5.5 Onan, Heated HT pads, Two Uniguard Awnings, Slide Toppers, 2000w PSW inverter