me and my friend had this same problem
if therer r any woods around ur house look for fallen brances or small fallen trees and move them into the field then u stack them on top of each other till it is high enough
or u could get some old wood and just make a bunker out of that a piece of ply wood w/ a 2x4 if front and behind it on each side ( this is my favorite home made bunker it works really well )
if u put that in the middle of the field so ity cna b used by both teams