Foam Sheet

6mm White depron is probably the best XPS (Extruded Polystyrene) you can use, as it is quite stiff. It is made in Germany. CLICK HERE FOR DEPRON

However, Jetworks Jets can be built from many different types of XPS sheet. Some are stiffer and some come in different sizes to the plans but with simple scaling to suit your preferred foam size, you can make your plane in different sizes! For example:-

5mm foam = scale down the plans and 3d prints 84%
9mm foam = scale up the plans and 3d prints 150%
12mm foam = scale up the plans and 3d prints 200%

(we do recommend skinning larger planes with a layer of polyurethane varnish and 0.6oz fibreglass cloth.)

Finding XPS sheets is quite easy. I recommend using Google / Ebay as a starting point.
Search words :-

‘6mm Insulation board’
‘XPS sheets’
‘Underfloor xps’