Heavy duty windowblinds locking cam cleat

I have a 70 lb heavy bag that I don't want to leave outdoors, moisture, etc.
The outdoor setup is 2 4x4s in the ground amidst 60lb concrete bags 2x6's on top spanning the 4x4s.
At the top center I have a bolt that the bag would hang off of.
Right now I'm throwing a rope over the bolt then securing the end of the rope on eye hook on one of the 4x4 posts.
Although I alternate posts, this means that on a given workout, one post is under greater strain than the other.
I would like to have some system like they use in the cheap windowblinds where I can pull the rope up straight into the ratcheting wheels and it locks in place until I'm finished then I can pull the rope out at an angle and have it lower down.
Anything like that exist?
I looked into sailboat cam cleats but when you pull the load bearing rope outwards it releases and this could happen during the course of hitting the bag. I'm not explaining that concept well but it seems (a heavy duty version of) the windowblind ratcheting wheels is more what I'm looking for.

 

I would continue looking at marine hardware. There are several different types of locking cleats. I would route the rope up through a eye bolt or pulley and then into the locking cleat. That way the rope is always pulling in one direction and it can't jump out of the cleat.

 

Why not just a regular dock cleat?