Hi Matt
I'm a newbie here and to e30.
Wtihout seeing any pics of the skirt, I imagine part of the skirt has to mount to the pinch welds (the standing seam) along the bottom of the rocker panel. Correct?
At the corresponding attachment points for which the skirt is to be attached, you can fabricate metal tabs and screw or JB weld it to the body. Placement of the tab will be behind pinch weld on the opposite side of the skirt. In other words, in a section view, the pinch weld will be sandwich in between the tab and the skirt.
If you are screwing it on, besure to add RTV/silicone sealant between the body and the tab
Materials for the metal tabs can be made from thin gauge flat aluminum bars from a hardware store.
Hope this helps and please post project pics. TIA!