Pet,
I plan to do the same with the batteries. I have a local retired welder who can do it. I plan to put some solar panels on the roof too. I might not get to it but I plan to. :)
Check out Harbor Freight for the cargo tray. They sell an aluminum one that's a name brand. I bought one last year but it's still in the box.
I found it.Be sure to look online for a 20% off one item coupon too!
Found it too!Edit, I found it again at $74.99 if you use coupon code 60620597. HF is maddening that way.
See if it comes up here.