Hmm, they make some intriguing water filters.
(maker of water filters, insect repellants, sunscreen, and more) also makes the micro/hollow fiber water filters. Except they're actually rated for pressure (if you dig through their documents)--the fibers can take 60 psi, the casing takes 40 psi, and their gravity-fed pressure charts top out at 20 psi. Plus they recommend backwashing the filter to unplug the fiber pores, allowing for a ridiculously long filter lifetime. And you can get just the filter for $30-40 (product sp129).
So that takes care of the most the biological concerns (bacteria, protozoa/cysts), but there's also viruses & some chemicals to worry about, plus sediment. I might get a UV light to kill viruses ($50-200) to put after the sawyer filter. There's a strainer (300 micron/50 mesh) before the pump, but getting something more like 5-50 micron would be closer to standard practice.
So one can purchase empty inline filter cartridges
, plus filter media (granular activated carbon/GAC, KDF 55 or 85), even wine filters from 0.2 micron up to 6 micron or such. I think it would be fun to make my own water filter, though it wouldn't technically be rated. Or I could just purchase an inline filter ($20?) for chlorine/lead/heavy metal removal rated for 5 or 20 micron, 1500+ gal and call it good for a few years (~6 gal used every two or so days).