"flush fitting" and a capacity larger than 7 round really don't work in a Sig 220
they did offer a different follower for the 7rd mags which increased the capacity to 8rds, but they are not a reliable as the current 8rd mags which have the extended base pad...made by Mec-gar.
Sig offers an extended 10rd mag, but as already stated, it does shift the balance a bit.
the other commonly used mags for the 220 are the 8rd mags from Act-mag....they're manufactured by former Mec-gar employees (sometimes branded as Novak). i have about 25 of them, but i come from the school that when single stack .45 ACP mags start misfeeding, throw them away and grab a new one. being as i got them for <$20/each, i just bought a bunch.
i have about 6 factory 8rd mags and really don't see much difference between them.
i have some 8rd and 10rd ProMags...i rescued them during a class...that i use for failure drills, because they hardly ever feed 2 rounds in a roll.
i also have 5 factory 7rd mags (dovetail, not zipperback) that came with my older 220 that i use for training as they will bring out flaws in my practice...due to their finish and lenght