I have a flat yard too but this summer i threw together a 7 foot tall drop in ramp at about 30 degree slope. I used 100 percent recycled wood i got for free so just look round for some wood then throw a cube together put a top on it and some diagonal stabilizers and put something for the in run and pack bout 2 inches of snow on the running surface and your golden. If you want i can post up some picks of my drop in ramp