What you want is slightly larger than the 50mm. Pall, Millipore, Parker Advanced, Meissner, Whatman, etc. all make 'capsules' of 1 sq. ft. of membrane surface area ... abut "fist" sized and come with hose barb connectors. Suggest you contact a 'distributor' not 'direct w/ mfgr' and sweet talk them into omitting the minimum charges / minimum qty, etc. that usually apply. Also you dont need a 'sterile' system filter, just a 'log reduction' value (LRV) @ 0,2µM, hydrophobic teflon or equivalent membrane. an example: Single-Use Capsule Filters - Meissner Filtration Products
Failing to obtain one from an ethical mfg., just take a fist sized wad of loose bandage cotton held in place with bandage gauze and a minimum of bandage tape to hold in place on a hose end; keep dry and change-out at least every 12 months. That will keep all the 'spores' out of the tank ... but tank, vent and distribution lines
, etc. still needs to be 'shock sanitized' yearly with clorox, etc . at ~40 oz. clorox per 100 gallons for 1-2 hours.
When you open the tank's inspection port, reach in and feel 'slimey stuff' (bacteria, etc.) on the tank walls, then you need to 'scrub out' the tank, etc. before