I always found a rock to put on the downslope side if I thought there would be some movement before I got the Hyd pressure up for the brakes. I always joked about if there were no rocks I would chock it with some of the luggage but that never happended.
As a note not in the flight manual but you could In a emergency throw the emergency accumulator valve and then you would have brake pressure to stop you if it was getting away from you on a slope start. This has stopped one or two agustas from rolling over the edge or into objects.
But I repeat this is not in the flight manual. and then I would NOT retract the gear but it would keep you from rolling off the edge of whatever.