Take a look at Defender and West Marine. Perko makes a huge line of hatch locks. A simple latch with a padlock will work.
My feeling is, there isn't really anything of value in the cabin, and I have insurance.
Lock = BREAK in
No Lock = Tresspass
I've kept my boat is not awesome neighborhoods, have never locked my boat and have never had an issue.
My key sat in the ignition of my 2800 for 9 or 10 years never being removed.