Tom I always put a runner on my plastic sleds. otherwise you simply cant deal with them on any kind of side slope.

I find about 6-8 inches of aluminum angle iron put on each side at the bottom work pretty good. just get a couple washers and round head bolts and bolt it onto the bottom on each side near the rear of the sled on each side so you end up with about a 1 inch runner that will cut into the snow a bit and keep it from going sideways.