What's a simple way to secure the companionway?

There are a few easy methods:

  •  Use a simple hasp hook or other basic latch that operates from one side. Then install a threaded access port in the top hatch board, positioned to allow reaching through the access port to operate the latch from the other side.
  • Drill a hole through the spray hood that you can put a peg in from inside or outside.  Pushing the peg in from one side pushes it out the other.