never had clue on how to make it (by hand)
where do you find info on how to make them from scratch?
I have been reading on how to form a mold to make fiberglass parts.
I don't have any better idea than you do.
IMO, if you go for the style in the GT-R pic I showed you, it shouldn't be too difficult.
The way I would approach this is first off, come up with a plan that you think will work. Come up with exact dimensions, how you'll fit the wiring harness in, etc.
You'll need to come up with a way for your backup lights to turn on. I'd recommend wiring a white light backup light into your 3rd brake light... That'd look cool and you wouldn't need a custom bumper! (The GT-R was a racing car & didn't need the white backup light.
) All you'd have to do for that is come up with some sort of new 3rd brakelight cover with white bulbs on the sides or something light that... Hmm...
As far as making the plastic the shape you want, I think it's as simple as heating it up until it's malleable then shaping it. You could use the stock tail lights "container" as a base... In the crappy photoshop I did below, the stock 3 piece was replaced with 2 larger pieces that filled the stock area and 1 of equal size that reached out onto the trunk lid. To make this functional, you'd have to have some sort of connector that relied on the trunk being closed & contact to be made.
This is a really cool thought man... It'll be a lot of work, but your end result could be AWESOME!