Questor,
The old H&R M158 or "Topper" shotgun action is an inexpensive utility weapon and relies on a rebounding safety. That is, the hammer will not stay in the "down" or fired position when at rest or after pulling the trigger.
I do not know of any device that allows you to safely carry the piece "cocked and locked".
Why would you want to carry the piece around with the hammer cocked? I would think that a fast thumbcock is all anyone needs for hunting purposes.