The spoiler hangs very low and I would be concerned as it might get damaged during shipment of the car. Maybe it would be better to mock up but keep inside of trunk during shipping?
In any case I use stainless bolts, nylock nuts and fender washers that can easily be bought at the local hardware store (get metric in David's case). I use 8 bolts. Mark the spoiler 8 evenly spaced spots and drill through the mounting flange. Use that as a template to mark the lower valance. All you have to do is place the spoiler on the forward most edge and clamp temporarily. Once the valance is drilled just line up the holes and insert the bolts. The plastic will conform to the valance once bolted up snug, not too tight though. If you have a nice painted valance I would use thin rubber washers.
I have never made any Challenger spoilers and yours is probably from Cudaspoiler guy as he has the fiberglass ones. Grateful you like my spoiler though thanks!