This should just about do for you, 80x40x4mm ali section:

http://www.ebay.co.uk/itm/Aluminium-Box-Tube-80mm-x-40mm-x-4mm-Wallx250mm-Long-/160326529256?pt=UK_BOI_Metalworking_Milling_Weldin g_Metalworking_Supplies_ET&hash=item255434b0e8

Should be strong enough for your use as it's 6000 series alloy.
Btw, good job with the testing, really interesting.