We used to use 4x8 sheets of plywood laid on the grass for Q500 takeoff points. Not sure if that would be long enough for EF1 or not - but if you have your helper hold the plane at full throttle just before the flag drops, then shove like a race plane, it might work. I think we also had maybe some 2x2 at the far end of the ply just to make sure the end wasn't digging into the grass to "trip" the plane right as it got to the end of the ply.