Originally Posted by tileman
It may be easier/cheaper/lighter to split the HDPE up into 2 pieces say 7-8mm thick, and then bolt them to the outsides of the factory brackets. Adding in some standoffs throughout will help keep eveyrthing rigid/aligned, and the 2 smaller pieces should offer a bit more torsional rigidity than the single large piece.
Would you be better off for your adjustments at all by slotting the hole closest to the instruments on the mount in a circular manner? It might let you keep the relative positions the same while fine-tuning the viewing angles.
Definitely looks easy to produce with what's available though! I'm a huge fan of the ADVMonster LEDs.