How to construct a ski/snowboard box?

For the top get PVC sheets or ABS sheets, get ABS if you can it has higher and lower temperature tolerances. Use a router, sandpaper, file, etc. to round over the edges of the plastic sheeting, this way you won't need a coping. Pressure treated lumber is not necessary, but you will want to use some sort of product to make sure the wood won't rot. For dimensions make it at least 10 feet long, any shorter and as soon as you get on it you'll already be off. For width I would say anywhere for 6"-12", and about 12'-15' long, go longer if you can but it starts to be unpractical after 15'.