I needed to take fresh water tank out of my KZ Sportrek.
The coroplast was thin and sagging. I found 10MM (.390") black coroplast at local plastic warehouse.
I supported it with plastic T pieces used for greenhouse's. Seems greenhouses use the clear coroplast for insulation properties and does not shatter!
It worked well and no sag at all!
I did find fresh water tank sagged when full so re-supported with a C channel I found at Homedepot, no more sag!
When weather improves I will redo whole underbelly.
Tim
