Bicycle Bob: I've read somewhere about vacuum-forming and it's not a bad idea. No matter which material will be used I first need to make plugs for molding purposes.
And I don't have trailer terminal.
aerohead: 0.257 figure sounds impressive and worthwhile. About wheels - I'm actually running on regular steel rims, these hubcaps are original FIAT parts but since one is slightly chipped they stay in my garage. I don't think there is a flat hubcap supplier here in Poland and shipment cost from UK or wherever they can be bought would probably bump up the price significantly.
Piwoslaw: I'm glad you like the idea. I think that a closing part would not fit in the trunk if it was in one piece and making it from multiple parts would probably make it flex a lot. Every additional kilogram of weight counts when it has to be supported with homemade mounts
I haven't thought about using rear lights from different vehicle but with narrow space like it is inside the tail I could run into legal problems - minimum spacing between lights is 60cm, also lights can't be mounted further than 40cm from the sides. I think that I could use small, trailer lights - shouldn't be hard to fit.
I don't think that license plate needs to be relocated with open tail.