Looks like a 2'x4' sheet of perforated 1/16" aluminum sheet can be had for around $90. 90degree crease and sandwich that between two pieces of thin ply and it might be adequately strong if then supported by bracing to the chassis. The sheet weighs ~4lbs too.
Now to see if I can source the pressure clips that hold the seat down to the chassis. I'd be satisfied with high strength Velcro in the back by the trunk, but want the front of the shelf held by the clips like stock.
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