Depending on how thick the hatch is, you can use old wardrobe locks. They are installed on the inside but have quite a long key, so a hatch thickness of about 30cm is not a problem. They are still available in stores.
Otherwise, an old deadbolt lock is easy to use. Many homeowners have such old locks lying around.
Interior door locks, which like deadbolt locks are embedded in the side of the hatch, can also be used.
Deadbolt locks/interior door locks, as well as the suggested wardrobe lock with its beveled bolt, are preferable for easily closing while compressing the sealing strip. Other constructions might require three hands.
Some tips - plenty of simple locks are available.
