Flat Floor materials
 

Excluding splitter and diffuser and going to the pinch seams we have about 2.3m by 1.3m or 7.5ft by 4.5ft dependent on your measurement desire.
Obviously most materials come in

1. 2440mm by 1220mm or 8ft by 4ft
2. 3050mm by 1500mm or 10ft by 5ft

Aluminium (or aluminum if you wish), alupanel, dibond, plywood have had their discussion but for those not in the US get sign material at those sort of sizes is expensive.
I'd like to snag a alupanel for a few bucks as most say but it is £65 ($88) for 8x4 sheet for me from a local sign place yet 12mm (0.5") plywood is £22 ($30).
3x the price for a 1/3 of the mass and half the rigidity doesn't seem that good to me but then again every ounce shaved helps.

What materials are you using for your underfloor (not splitter or diffuser)?
How much was it in local currency?
Where did you get it?

This is my current listed information
Attachment 222215

Revisions:
201805220354 - Added 6061 Aluminium 1.5mm to link with the 1.6mm 6061 used by 1993ka24det flat floor.
201805220355 - Added 0.9mm as it is the only one that gets close to Ryan's costing ($3/lb) flat floor. It feels too cheap to be useful but as its secret sauce it is unlikely to be disclosed.
201805220356 - Added plywood and Alupanel (similar to DiBond)
201805220357 - Added AluFlute
201810151159 - Added Young's Modulus to everything I can source. Removed AluFlute as its no longer available.

Reserved for clarifications

And it strikes again