and easy solution is to make up some short stainless wire pieces with swaged loops in their ends. Install a couple of strong hooks at the bottom corners under the lip of the companionway
and an eye in the middle of the hatch. Hook the wire over the hooks and padlock it to the hatch eye (keep the key on a hook just out of reach). The wire will form an upside-down V and should be tight enough that you can't get it off the hooks or push it aside to get through (you could add a cross piece of wire in the middle if the gap is too big).
This arrangement keeps someone from pushing the hatch open or yanking it up off the slides. It's also easy to store.
Of course, you can cut the wire with the right tool but as anyone knows who's made up rigging
- this isn't easy.