Hello folks

Going backcountry camping with my gf and another couple for a 4 day portage trip. Im the only one with any experience/gear for this kind of trip and thus am the only one with a water filter. I have a sawyer mini that i was hoping to utilize into a gravity filter so we can just constantly be filtering water for everyone with little effort

I was thinking of just bringing an empty 2l pop bottle and using that since the sawyer has threads that match the bottle but then i got thinking about what could work better, and a proper bladder came to mind. They seem to be somewhat expensive though (~$50) for what is essentially a plastic bag with a tube coming out of it

Does someone know where i can find a cheap one in canada? Or what else i can use that would be cheap?
Are bladders just expensive and im going to have to nut up if i want a proper one?

Not opposed to buying some tubing from a hardware store and diying the shit out of this, just dont know what to use to hold the water other than my original idea of the pop bottle