I would definately say cardboard would be way too unstable, even if placed on a very flat bench/table surface. The best type of board I've found is the back side of ceiling tile(sometimes called "Celotex"?), again placed on a very flat, sturdy suface... and do check that its flat..thats probably the most important part of the whole enchilada..

If you'd like to look and see what I'm talking about.. you can get them at any major hardware store..IE..Homey depot, Lowes, etc.. and the last one I bought was about $5.. and it comes in 24"x48" sheets. One sheet is really all you need, for most .40 size planes.. as the wing panels are usually built separately(one half, then the other).. and then joined(which you dont need the ceiling tile for).. so it gives you about 48" to build a panel that is .. say 32"...(on a 64" or so span). I easily built 80"(overall) wings on one 48" panel.

The back side of the panel(non-texture) allows T-pins to easily penetrate, but still retains them well... and you can pin your plan right on it.. then cover the plan with celephane, or wax paper(easily found in any ladies kitchen).. and another nice thing is when your done with it.. what I did is just found a little slot in the closet and shoved it in there.. I've had the same piece for years and years.

Get some T-pins from your local hobby shop.. usually around $2-$3.. and that would probably give you a good start. Alot of things needed to build one of these is usually in any garage or household... so... any other questions... just hollar..

I too like the 4-star planes.. I've not built one personally, although I've built several Sig kits.. I have done final inspect on a fellow members.. and test flew it.. They are a straight away flier for sure.. very stable.. but still a very wide flight envelope.. and knowing its Sig.. well.. they've been around longer then me I think.. so .. good luck with it..

Thats about it for me.. there are other ways to create a surface..I've heard some use drywall.. I dont' like the chalky residue though.. .. but there are other ways.. but this is my 2cents.