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Thats not a solution, but a quick workaround (I am busy)

In the minkowski() use

cube([2Wall,2Wall,2*Wall+X],center=true);

where X adds more to the bottom. (not sure the whole X value or the half, please try). The entire object height will grow by this value (or the half), so you have to recalculate the Height too.

Maybe a nice other one with more OpenScad skills can fix this more accurate.