So i'm looking at developing a ducktail style spoiler for the mx5 mk3,

i have excatly what it should look like in my head, and there's the problem! how do i get from this idea to something phisical?

firstly i will need to get accurate dimensions and shape of the boot lid from the car, the finished product needs to fit snuggly to the compound curves of the panel.

i'm sure some sort of 3d laser scanner exists, scan the car, import into autocad or similar and go from there, but this is out of reach for most of us.

I guess the another option would be to take a mould from the actual car itself then use this to make a plug?

I thought you just wanted it to put your ideas onto the screen. If you want to properly design a part and then use that design to create drawings that you could make a really accurate male plug from then you really need to get hold of a proper CAD program like AutoCAD or Solidworks, I don't know much about free alternatives but these programs won't be cheap at all.