If you're using gas shocks that just assist with lifting the hatch, or aren't strong enough to hold it up in all circumstances (wind, snow, whatever), I saw a simple way to lock one side in the extended position without needing a separate prop mechanism:
Find a piece of metal tube that is sized to slide easily over the body of the shock, length 1" greater than the minimum rod extension you want to maintain. Cut halfway through the tube at the desired rod extension length, and remove 180 degrees of the tube wall, leaving a lip that extends 1" beyond your desired extension length. Drill a hole through the tube so a locking pin with wire retainer (like McMaster's 98416A211) can go through both sides of the tube (the tube also needs to be large enough in diameter to accomodate the retaining pin and the rod at the same time, the pin should be near the top of the tube).
The idea is when the shock is extended, the retaining pin traps the rod to one side of the tube, so the shock will rest on the tube wall when under load, thus holding your hatch up. To close, remove the pin, lift up on the hatch a bit, center the tube on the shock (simply push the tube so the 1" lip is against the shock's body) and then lower the hatch. When opening the hatch the tube will tip to the side, blocking the shock so you can insert the retaining pin without having to support the hatch with your other hand.
I'll post a CAD drawing tonight to better illustrate the concept.