[ins0mnia24]

That would be a simple build the easiest way to do that would be
Go buy some Oaktag from Staples or where ever you can get it cheapest
Make your pattern and shapes with the Oaktag get it all cut out and tape it together so you have a pattern of the size of it and how it will look and see if it fits where it is being installed ect... Dou you happen to have a picture of what it should look like?? If so post it ..
The only trouble i see is getting 5mm Thickness that is really thin and structure wise Fiberglass resin by its self wouldnt be strong enough
What you could do is take your taped together Oaktag pattern then get some fiberglass mat and cut it out so you can apply it directly over top of the Oaktag this well act as a Form or Mold so you can retain all of the bends and shapes needed to build the box Once that dries you can take a 2 or 3 inch paintroller with fiberglass resin and roll it right over the Oaktag this will build a protective surface over top of it. then your pattern would be completely covered in Fiberglass
Normally i would use Carboard as a backing Mold or Form but since you need it so thin the Cardboard+fiberglass mat would probably be to thick so i would use Oaktag.
I know the Oaktag and Cardboard sounds iffy but trust me it works very well
I have used that technique on many things from Race seats,Dune buggy seats , Custom kicker boxes ect... Once the Fiberglass dries and cures 100% over both sides of your pattern it will be Very strong
And plus it is a fairly simple process just makes sure you have proper ventilation because the fumes are pretty bad and hazardous make sure you wear a mask.