Since my first attempt at making molds out of fibreglass was a learning curve, and I must say a lot easier than I thought. It gave me the confidence to make a mould out of a pattern, (buck) I have been working on and off for about 8 years, between road crashes, illness and earning a living.
Thought you may be interested to see how it turned out. By the way, its a little larger than the spoiler.
This is pattern, (buck)
This is the mould and the finnished roof.
In the past I sent a pattern to a professional mould maker. The mould was exellent but it cost me a lot of £, $. With my time and doing it myself (diy) I would say its cost me a third. Hope you like. The lesson, do good research and have a go no matter what it is. Do even better research if its brain surgery your attempting.