I can fit 2 flooded group 27s in my under body battery box, longways, but might have to modify the handles somewhat to fit.
I have an acess hatch to get at them from above.
Mine was made from the garage door angle iron which has all the holes predrilled in them. I was originally planning on attaching some sort of retail tray and fasteners to frame, but my friend with the welder said we can make it for a fraction of the price with materials he had laying around, and weld it to the Van instead of bolting it.
I think since you require a welder anyway, well most welders also can fabricate everything and might prefer to do that rather than trying to weld in something prepainted or which was designed to be bolted.
The fabrication of everything route opens up more options in terms of mounting and location, in my opinion. With 4 of those batteries you might want 2 on each side to even out the weight distribution.
I'd find the welder first, ask their opinion on mounting your AGMS on their sides for minimal ground clearance loss. Let they think about the design they would like to make. Do have them consider rocks lifted up and thrown by the front tires, and perhaps rear tires if one is trying to back up in gravel up a hill.
One thing my thermocouple indicated to me when I installed it on my underbody battery is just how much engine heat goes under the van body when moving. Not sure what can realistically be done. I incorporated a heat shield as the exhaust pipe was 18 or so inches away, but the readings I got when driving were quite high, and dropped quickly when stopped. Prolonged driving could bake the batteries, and if high amp recharging via alternator could have them hotter than they would like to be