Start looking up scaffold companies in your area. I'm sure you can rent out the stuff you need instead of having to buy it. Basically what you would need is:
Then you need a few extra of these for the ramp:
I would get some plastic lattice like this in 4 x 8 sheets and use this for the sliding surface for the flat platform and the start of the drop in.
Then make the tranny and lip out of natural snow.
You could also try laying some kind of cheap carpet down on the drop in ramp and throwing snow on top of it. It should hold well enough to make the carpet fast enough, but it will also add weight. The last thing I would want is to have the platform collapse with someone on it. That's why I would go with lattice as much as possible. It's lighter and it is super fast.
Hope it all goes well.