have to agree with Pukka re the cardboard, works well
i use proper PVC foam sheet, usually 6mm, between the laminates and makes it mega stiff with very little weight addition. cheap enough too from glass suppliers. in fact its less than cardboard as it doesnt really absorb resin (the heavy bit of the process).
another really useful product is 3mm foam core. ive made a couple of bonnets and panels using it now, very very light although about £5/m2. does help make the panel quite resilient too. its full of perferations so you have to be careful not to use excess resin and 'soak' the core, best to lay up the resin with a pound shop paint roller rather than a piant brush to get a thin even light coat.
as an example a std fibreglass scimitar bonnet is 15.6kg, the ones i made weigh 5kg with no aramid or carbon
Aramid fibre cloth (kevlar) is mega resilient but a bit of a bastard to work with. it doesnt 'wet out' easily, and doesnt go round curves easily and defo doesnt do corners! real sod to cut too, you need shears rather than scissors and when its had the resin applied and cured, its a cutting disc in the angle grinder really. doesnt sand either.