McDave wrote:Hey Grummy,.....Hope you don't mind if I copy your design, and Thanks! If you have any advice or helpful links I am all ears.
PS, Could I bother you for rough dimensions, please? I had better start looking at tanks to fit in there.
McDave
COPY AWAY ! That's what forums are for, right ?
I had a specific goal of getting enough floor length to for two group 31's, though, if I end up with dual batteries, it will probably be two 6 volt'ers. The overall dimensions I ended with are roughly a trapezoid that is 30" long, 16" high and 10" deep. The trapezoid aspect leans about 5 inches forward, so one has to keep that in mind.
Now, I ended up purchasing a 68" long fender because some others I found that were longer had crazy shipping costs. And because I was thinking about two group 31's, I used 38" of that fender up by the time I had appropriate overlap on the front. So, the other side when I get to it will end up a little shorter, but I'd bet no one would ever notice. I also considered putting the WHOLE dual tire fender on and simply slipped in the rear vertical portion of the original, but I have that rear outlet in the way. In retrospect, I could have covered the outlet... just left it inside the box and left a hole in the bottom for cord routing... oh well.... no shortage of "in retrospect" on a guys first cargo build.
Of course one also could set the fender lower than the front one if those gains are necessary. I kept mine flush with the existing because when camped, I always dreamed I'd put one of those table hanging brackets on the side to set things on. Now with this fender's added length, I wont need to drag yet another thing along.
Overall strength considerations are a MUST here, and one would need to do a few calculations to see if water would end up being heavier than two batteries in the amount of space given. Of course there are always ways to make it strong enough. I was actually surprised to see just how much the fender itself did to add rigidity, even though I only was able to catch two vertical wall joists. I have screws in mine currently, but they will be riveted.
Of course UNDER supports are key. I made two vertical supports that are now riveted into the floor joists, the frame perimeter, and the box itself.

What you can do comes down to what you have for a trailer, where the floor joists are and or whether you need to also attach to the trailer floor itself. I and a neighbor both stood on mine when finished ! So a combined weight of 400+ pounds really did not flex much at all.
It will be interesting to see what you can find for tanks. I'm sure there will be something out there, albeit square. I am also looking for water tank options but have been focused on something that will fit between the floor joists. Everything I find is either too small in volume or too just too large in height. When I get a little farther along and can study what and where my weight concerns will balance out, maybe I too will be considering that other side for a water tank.