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http://snowboarding.transworld.net/1000095158/how-to/how-to-build-a-drop-in-ramp/ i know there was a video that showed you how to do this somewhere, but its basically like this.
make a platform to go on top with 2x4 attached to plywood. then make the support section, but instead of putting in screws or nails on the pieces of wood connecting the front 2x4 to the back ones, use bolts and screws so its collapsible. then, just make a the drop in out of plywood and 2x4s and attach some metal hooks on the top so you can hang it from the front of your drop in.