Securely pegged down strips of white tarpaulin?
We tried this once for a spot landing competition - the first aircraft nearly came to grief when it ripped the tarp from its hold-downs. After that, we moved the marker to the side of the touch-down area.
We used to use white tarps for marking the 'box' at aerobatic competitions so it does work for conspicuity, but they need frequent changing and washing.
Runway markings need to be non-hazardous to aircraft (sounds obvious but see above!)
TOO