I have a fluval 406 and used two bags side by side as the trays are divided. I purchased 100ml purigen as it is the only way to get it pre bagged. Two bags, one on each side of a tray worked for my filter. Just make sure you rinse them well first.
If you have the money and going to be using purigen for a while, it is worth it to purchase a couple extra. This way while you are recharging you can just switch them out. Also worth noting: Soak in water with Prime for a period after recharging! Make sure to read up on it. Another tip, if storing spare bags which have been used, keep them wet. A ziplock bag after soaking them would work.
If you are going with bagging it yourself, I would use the bags from seachem. Purigen is really fine and I have heard stories of it working through pantyhose and other bags.
Just my .02