Ok then I would use the 6mm polycarbonate and then just drill a load of 10-20mm holes in it, should be able to remove around a third of the weight of the panel and shouldn't affect its 'armouring'.
Alternatively just get 4mm polycarbonate (would weigh 215g for the lid) or 1mm titanium would only weigh 200g (you can get a piece the size you need here: http://www.fingertechrobotics.com/pr...?prod=titanium only costs £10 but shipping is £10, let me know if you do order and I'll get some to and split postage if you'd like) you could probably even get away with the 0.8mm or even the 0.5mm of you really needed the weight elsewhere.

What are you currently using as a baseplate? Perhaps that could be replaced with sometime lighter as well?