I had some aluminum sheet and built one like the Columbia Overland one - or as close as I could based on photo off the web.
Here is a link that would have helped when I built mine...
Discovery II - Homemade D2 Dual Battery Box - I have seen the Columbia Overland version and liked it, but not the price at $189 now plus S/H from the northwestern US. :) I finally got to measuring and drawing and making a pizza box version, based on the original battery box, my group 34 battery...
landroverforums.com
Not hard to do - took a afternoon or so of farting around and drinking beer.