My suggestion would be to place an orange cone in the middle of the doorway when you drive out. Then you have to get off the tractor to move it and can fold the ROPS down when you walk back to the tractor. I'd use a sharpy and write 'Fold Down ROPS' on the cone.
Depending on the situation, I use some small plastic cones for this when I have a car on the lift. One cone says 'Do NOT move' and I place on the windshield. The other is 'do NOT start' and I put it on the seat.
It wasn't supposed to be for me, but in case my wife or a buddy were to wander in the garage. But its been a good reminder when I've had to leave a car in various stages of disrepair for a few days. 'Did I put oil back in this....?'