The magazines for Mossberg’s 590M magazine-fed shotguns (and similar) are massive and are unlikely to fit magazine pouches made for other magazines. Wilde Custom Gear is currently developing a magazine pouch to fit these magazines.
The image they shared (above) is a prototype but you can already see the same features that Wilde Custom Gear uses on their other magazine pouches. The pouch features a laser-cut PALS grid on the front, routing for bungee retainer cords, and a fit that should allow for compatibility with multiple magazines of this type (single stack shotgun magazines).
Stay tuned for additional details.