Combining multiple box colliders into one edge collider

I've got a parent object which is made up of a bunch of small 'block' children objects. Like this:

As you can see this object has many colliders, and the ones in the middle are not really required

Due to the way some other parts of my game works I need the object to have just 1 edge collider surrounding just the 'actual' edges of the object, like this:

I made that edge collider myself, as you can see it's pretty rough and takes a long time to edit manually by shift-clicking the collider vertices into place.

Are there any ways to do this automatically?