APS CAM870 Top rail mount

I know there's a rail mount for APS CAM870 but I don't want to spend extra money for it.
It also damaged my receiver because of its top rail screws.

If your CAM870 has rear sight on the receiver, yours isn't compatible with this.
(I'm not sure it's remove-able or not. My base model (CAM870 Douchebag) doesn't have any iron sight what-so-ever.)

There's a tiny notch place at the trigger group. That's where you have to lock it. (note 2nd image above)
This design has somewhat tight length in order to lock in place. Expect light hammering.