I made a box a few months ago...
I made my measurements so I used every inch of the 2x4s so these aren't all exact.
2' Wide <--Way to wide.
8' Long <--Too short but you its portable and I can always make a second one to add.
1' Tall <--Perfect. Just build the snow to size.
Holding the surface is the 3 joist coming from the sides that attach to the sides and I added 2 more. Err, maybe I'll sketch it up tomarrow.
I used 3/4in ply for the surface base. I suggest you go like 3/8s or such. I just had it laying around. You can see I pieced together the corner.
The top surface is actually waterproof walling. It was like 15 for the sheet. Its very thin, I cut it with a blade. It looks filmsy, but I just took candlewax and rub it all over. Once you get it a little wet with snow and what not, slides well.
The pipes are basic pvc. Heres the pics. Sorry I deleted the ones without the ply on...
Don't build it downstairs. Its a pain to get up by yourself.