It looks great! The side-locks make it exceptionally nice. I'm thinking that you plan to post pictures of the finished product as well and I look forward to seeing them. I've attached a picture of an original box-lock that I have configured with 16 ga, shot tubes and what is for all practical purposes a .22 LR running through the rib.

Tinker hit it on the head calling it a "Schienen" for rail or rib drilling.

Regards,

Buchseman