Slept on it. Why not just alter the existing magazine to single feed? There is a lot of good engineering in it that you just won't waste, and it will mainly involve some brass soldering, but you may of course not get as many rounds into it.
Blu tack on centre line of .303 magazine mag follower, stick a 410G shell on top centrally, try chambering it with the bolt. Once you get the correct height you know where to solder on the lips.
Alternatively, looking at the Sterling SMG mag above, you can straigthen the current feed lips on the SMLE mag and add a pair of double feed lips that have the top round riding significantly higher than with a current SMLE mag. Would save you interfering with the magazine follower (though it would now still become a bolt stop when mag empty) .... and this might even give you an extra round magazine capacity, to offset any capacity lost by using the original mag!
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