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This Commanderwagen has a field modification that is a roller for the front valance. Something like this could establish a low stagnation point.
Or maybe a high stagnation point and prow like a boat?
My current car is as low as I can practically get it, but for the next one I'm leaning toward a high ride height and heavily skirted and spatted wheels. A big problem for you will be the wheelwells.
For the belly pan, consider
Grimco - Substrates: Max-Metal, MAX-metal HD. It's like Polymetal or OmegaLite, but the aluminum skins are .30mm, about twice as thick. It's available in 4x8 and 6x10 sheets.