I studied pics of a single stack 308 magazine, and where the lips cradled the shell. The shapes were made by carving an end of plywood and using a rubber mallot very gingerly. When I estimated the size and bent the first prototype, I loaded a shell and slipped it up into the blind magazine area(the action was off the stock) and seen where the bottom half of the bolt was just skimming the lips and still solidly pushing a round out. This depth also marked (at the bottom of the existing blind mag) the exact ending point for the aluminum channel. This acts as the new magazines stopping point. Sideways is being done by a bottom plate recessed into the laminate wood. A spring loaded L shaped piece of aluminum is my mag holder in place. Too complicated to write I'll post pics soon. I used shipping strapping metal bent in Z shaped intravals to create my spring for the follower, and flat alumium with countersunk nut to bolt strapping metal and follower. All is made so I can remove only 2 sheetmetal screws(front and back of mag) to remove the U channel and pull follower out. Sorry I got long winded.
Dremmel tool setup for brushed circular effects on the aluminum channel.