flite test designs are designed for their custom foamboard. If your in the USA, you can get Adam's Ready board at a dollar store, which is the closest 'off the shelf' material. it is 5mm thick with the paper. The only other name brand that can work well is ROSS which is only about 10-15% heavier. Most/all of the other foamboards are about 2x as heavy as the Adam's Ready board, which will make it quite hard to get your CG correctly without adding a lot of nose weight - therefore making your craft very heavy.
if you don't have access to DTFB (the Adam's Ready Board) or FT's foam, 5mm deperon skinned with tape can work well. Typically you will only need to skin the outside of the foam and can leave the inside untreated. (for some of my builds I pull all the paper off the inside and I haven't had any durability issues from doing so)