Just grab any side of camera and create new guide. For removing, move guide outside camera.

Setup guides numerically.

There ar 4 types of adjustable composition frame - Free, Margin, Proportional, Square

Guides data are saved in Camera Data it means for every camera you can have different collection of guides.
You can also copy guides from active camera to the others.

Hotkeys as well as colors of guides are changible in addon Preferences.

 Important note 

Guides are Gizmos - to see them Show Gizmo button have to be switched on.

Changelog:

v. 1.0
- Initial Release 

v. 1.0.1
- Default guides opacity changed to 0.5
- When guides are locked in view - also possibility creating guides in view is locked

Enjoy, and feel free to contact me with any question or suggestion!