Yes, those rotary molded battery boxes aren't cheap...you might look into using a rotary molded water tank in the size you need and make it into a 'battery box' - they are more compact and might fit in your space.
One comment...after installing your batteries, make sure that they are braced inside so that they don't move around when you're on the road. It doesn't take much to abrade a hole in a battery case if there is constant movement...and if a couple of stones get inside, it'll happen before you know it...those are expensive batteries, protect them. I have even used that spray foam you buy in a can to secure batteries inside a box, works great, and can be scraped out when it's time to replace.
Ray & Pat; Morada, CA