Imagine a single shot stock, no inlet for the mag box so it's just an action diameter inlet from front to back. Put a repeater action into it and the stock is like a tray where the mag/follower would ordinarily be. You may have to shim/build up the bottom of the tray a little inside the mag cutout in the action so the cartridge sits a little higher and slides over the feed ramp smoothly. You could do it with some epoxy/resin/etc. but I think I just used a strip of 1/8" thick foam tape on my 6BR.

Get a piece of pipe/tube/bar that's action diameter and a bit shorter than the action. Add your weight to the mag well and then use the pipe to bed over top of it. Now your weight is held in place and you have a nice smooth tray where the mag inlet used to be.