Push as much snow under the trampoline as you can to give you as much material as possible for your fort.
Mold the snow into walls that reach up to the bottom of the trampoline so the trampoline acts as a roof. Form the walls all the way around you, but leave a clear space for you to get in and out.
Spray the snow with a hose or a spray bottle to help the walls freeze and solidify. This makes the fort last longer.