Like this. Fab up a single wall tank, weld a sump kinda like this, then cut the floor so. Peaks out. Then weld in a lid over the tank to make it flush to trunk floor and vent the space between tank and lid. That way you incorporate all of the features you were looking for but maybe a bit more effective of an end product.
Or just buy a premade tank to fit your needs and install it.
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Of course, someone already makes a tire well fuel cell...
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