some of the aero testing i did the other year.
@120mph
i would remove the original front undertray and make a new full one out of 12mm marine ply, then mount it underneath your current one. I would mount it on the original rear 3 bolts and the 6 m5's on the sides, then for the front part i would use some kind of brace from the crashbox.
you will need to make it strong enough to stand on.. when i made my first one i stood on it, circa 14 stone at the time.. and at 145mph the wind pulled it down enough that when i stopped i couldnt see anything wrong when i looked and it was only above 60mph did it start to bend down. it turned out that the fixings i had used came through the clam.
now my airdam is bolted to the clam with M8 bolts and huge spreader washers, and the splitter is just attached to that via rivnuts into the carbon.. i didn't think it would be strong enough but i can stand on it and its fine. I will brace it from the tow eye at some point.
this is as big as my splitters are.