there is some snow and i dicided to make a box. i got everything but dont know which material i should use as top sheet. My budget is about 30 euro and the box is 9 feet long. Also i need a run in and since i dont have to much space and the terrain is totally flat the inrun cant be so big. I was thinkng about 6 feet by 1.6 feet and 3 feet hight which material should i use for thee inrun?