Team MedBots

[Max]

For anyone who doeant know this is a (very blurry) picture of my robot centrifuge:
[attachment=0:33v81jaz]centrifuge pic.jpg[/attachment:33v81jaz]

It didnt do very well at the champs so i am planning centrifuge II. I have weighed the top pannel and it came to 4.5 kg and i reckon the bottom one much be around 3kg meaning i have 7.5 kilos just in base plate and lid (and invertable wheels). the new version wont hav ethe motr sticking out the top and so wont need the wheels so that should save some weight to go into the bar. But i want to try and save as much weight as possible to go into a heavy fan motor and a much heavier bar. So i want to change the 3mm steel lid and base to a different lighter material. It needs to be strong because it has that massive overhang going to the bar and so will have a lot of force going through it in impacts and just from being hit by other robots.
My question is what material shall i go fo the new lid and base? ive thought of polycarb but dont know what thickness, or 10-15mm hdpe or some aluminium but i doubt that will be strong enough. I have thought of titanium but it seems very expensive even at cheap ebay prices but i am open to suggestions on other materials.
thanks