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