I am building my own also, but I'm not putting it in a box, I'm building it directly into my Class-C using 16 200AH prismatic cells and 2x120AH BMS from BatteryHookup. Cost was about $1,600.
So far I've top-balanced and compressed the batteries, and figured out where I'm going to mount them. Fortunately I found a spot inside the living area that could accomodate all of the cells together in a single row. It is underneath the refrigerator - had to build a 3" platform to raise the batteries up over some copper pipes that were running through the floor (low point drain). Had to lose one drawer (no big deal) and will have to enlarge the hole under the fridge (also no problem, I will fab a panel to cover it).
Overall, I'm pleased. The LiFePo4 battery weighs 160lbs and will replace 240lbs of AGM batteries, so I'm shedding 80lbs but gaining a lot of capacity. The cells will be configured in 2P4S2P configuration for 800 amp-hours at 12V. I don't really have any loads that require this much power yet, but I have a Pioneer 12KBTU mini-split on order that I am hoping to install, and at that point I hope to be able to support limited amounts of off-grid A/C. There is already 900W of solar on the roof and I am going to add a couple more panels if this all works out.