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?

why dont you research all the different spoilers available for the mx5 and find a ducktail type spoiler that you can cut/modify to the one for the mx5. join them up make adjustments with body fillers, smooth it all off and take it from there regarding making composite replicas of it.