Hi - I had a similar issue with a Model 12 with Accutrigger when bedding it - that internal magazine wouldn't come out for pie.
I ended up wrapping the whole magazine assembly (front, back, sides, and bottom) in a layer of plasticine, making sure that the entire layer of plasticine was thicker than any part of the magazine, to ensure that when I was removing the stock after the epoxy had set up, no epoxy could be locking in the magazine in the well.
My magazine had a few holes in the sides, which was useful for making plasticine "rivets" to hold the larger slabs in place. Not sure how well it'd work with yours, since your mag seems pretty flat-sided.
If you're going to try this approach, definitely do some action-to-stock dry-fittings with the plasticine in place before you do the real run. In particular, during the dry-runs, make sure that when you pull the action back out, the plasticine is still in place and hasn't been moved/thinned-down by bumping the mag cut-out on the way into the stock.
It worked out pretty well, but the whole process was pretty nerve-wracking - I couldn't find any references anywhere on bedding an internal magazine action with the magazine in place, so I was forced to make up the process as I did it. Result was worthwhile, tho - made a solid 0.5 MOA shooter out of the otherwise stock Savage BVSS.
Anyway, good luck with this!