Welcome to our forum. Many suppliers of suitable 9mm or 12mm ply will make at least 4 cuts free for you. 8 x 4 is an economical way to buy, but unwieldly to transport home, and accurately cut., but which way is "in half" 8 x 2, or 4 x 4, which will fit through the loft access. I like green, and maybe paint your favorite track ballast colour where you plan to lay the track. think about stiffening battens, 2" x 1" is a good starting point, with the board raised 2", with the 1" edge screwed and glued to the board. Just for reference the third radius curves are about 1045 mm, or 41.2" diameter to the outer edge of the sleepers. Enjoy the journey, that's a great set.