Ideas for a homemade backyard airsoft course?

Well I've just got into competitive serious airsofting *with aegs that shoot 300 - 450 fps* and since I'm tight on budget, I can't afford to drive to sc village in cali there and back along with paying 25 dollars each time I play. My friend has this huge *** flat dirt terrain back yard and we were planning to set up our homemade air soft field since it's consider private property and will save us money. Already we've managed to get an abandon car in the field. Maybe we can use alot of cardboard boxes as bunkers/shelters. Reply with some ideas/tips/opinions. Best answer gets 10 points