you should use HDPE, thats the same kind of hard plastic they use on real boxes...
its also the stuff they line ice rinks with....
or if you don't have the money or know where to get this; attach some PVC pipes to the top of the box....
for further information, look into the "build a jib" cult
you'll be amazed how much info and how-to stuff you can find in there !
good luck and have fun !
"The joy i get from skiing; that's worth dying for !"
-CR Johnson, RIP
Belgium represent !
GoPro (HD) Cult: join now !!