Finally i managed to get the rear box done. It is a 920mm x 620mm x 300mm straight box made from stainless steel. Yes stainless! I decided to use that since my Mazda rear box from mild steel started to rust after 5 years of use.
I welded it into car trunk after i cut out spare tire well and some of the trunk.
The box was made with some overhang. I made holes there and i used those holes to spot weld the box in the trunk.
I used MIG welder which has good temperature control. But still i managed to burn the isolation in the rear bumper!
I drilled a hole at the lowest location to drain in the case water would come in.
After welding i grinded the spots to even height and i protected the welds by degreasing them with Nitro and then sprayed zinc coat on it. Since box is made from inox i only used zinc on welds. The rest i will paint in epoxy paint.
On the outside i also welded reinforcement under the box so it will be able to carry the battery weight.
In the rear i also welded another reinforcement so i will not damage the inner box if i hit something in reverse.
leafdriveblog 
You forgot to add the pictures.
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