Kind of an odd question but where would I look if I wanted to have a groove milled into the backside of a picatinny rail section?

Basically I have a number of different brands of spare rail sections that attach via screws and I need a channel milled into a couple of them.  I dont have the exact measurements right now but basically a 3/8" channel milled the entire length of the rail on the backside of it and obviously centered.