[without melting and contact cemented it to the tray]
All depends how you want the resulting product to be.
I've worked many years ago in a small polyester boats and canoe factory and did over the years many parts.
for my Mustang did a custom ram air, front valance...
What you see often on you tube is to shape with foam or clay, then lay some resin/fibers and sand this.
So far so goed, but at this point the question is how good you want it?
Most will then sand the construction, cover with some aluminum, roll some resin and consider work done. You see this also used for interior parts.
You end up with a product that isn't very strong and likely to bend overtime.
That's literally 1/2 work to me and your project can't use something like this! And its a spoiler, it doesn't take much speed to break it if its not a strong piece.
Looking at the design you want, it may take the valance with it too.
On the shape/skin itself, here's a few things that may help you:
Once you have your shape done (checked the sides are symmetrical with cutouts along the shape at regular intervals) and sanded it,
You actually have nothing but the negative print that will define your mould. This because you prolly want a light and robust spoiler, where you can define the thickness
where the stress is likely to be the strongest.
So instead of painting, just cover it with loosing wax, let it dry and reapply a few times until the cotton cloth "flies" over the surface.
Then add a layer gelcoat, of say 3 mm. When dry apply a thick layer of resin + polyester cloth. in between add some metal tubes or whatever so that it doesn't twist
later on. When dry disconnect it from the original shape, you do not need it anymore.
Then its time to redo; you put loads of wax in there, black coat probably as you may it the road/something at some point, Add the resin + mat inside, and define the thickness the most regularly as you can. If you have no vacuum pump, its better to add the resin in layers in 2 or 3 passes vs a thick layering that will likely have bubbles.
Its dry and time to separate the shapes. If you have thought your shape right, compressed air or a small lever will suffice to do so.
However this technic requires convex shapes and also assumes the entire shape can be disconnected from the mould going one direction.
As per your design, as you plan sides, if the base near road is 1/2 round, you can do this in one go. otherwise, you will need to make two moulds and assemble
the two positive parts together.
That's just the skin. What matters more is how this skin is actually hold onto the car. You need to have a construction, a bit similar to the bumper, where at least 3 very strong ankers. Looking at your pict, I'd say you need you have a tubing construction that goes under and behind the valance. Then just like the bumper, you need
a way to bolt it. Once you have that defined, you will then have to add these connectors to the backside of the skin so they become one.
It is also likely to me, that you will have to add a flat surface that will allow you to connect it on the valance without gaps.
There are many ways to go to Rome, so I will not extend more as the shape/structure behind will likely define the way you will proceed.
Just wanted to give you few hints that may (or not) help you make the front as kickass as the back!
