What kind of material is it and where to buy such material (possibly in Europe - Czech Republic) for it for such nice strach build planes, please?
The stuff is called foam board. The brand that flite test uses isn't available in europe as far as I know (I believe it's called Adams Readyboard, but feel free to correct me.
It's basically a layer of ex
truded polystyrene foam sandwiched between two layers of paper, an overall thickness of 4.5mm and 5mm will work fine without the need to modify the plans.
Stuff like this is primarily sold in arts and crafts stores. The use i knew it from is building architectural models... so if you know someone who's an architect or studying to be one they might know where to get it locally.
I'm not to sure about online sources outside of germany, although I believe
modulor.de and boesner.de(sorry, can't find it on their shop page at the moment. I'm sure I bought some a while ago at my local shop...) will ship inside all of europe. The price is significantly higher over here, though.