I picked up a Rubbermaid FilterFresh water bottle and filter today for $7. The bottle only holds 20 oz, but is widemouth and will work with a Steripen. The filter fits under the cap like other bottle filters, but has an incredibly high flow rate (because it is NOT meant to filter microbes, but the packaging says it will remove chlorine).
It also will fit on a Nalgene bottle. The threads are ever so slightly different, but I screwed it onto a Nalgene Canteen and gave it a good squeeze. No leaks. The odd part is that the Nalgene cap won't fit on the Rubbermaid bottle.
Wet, the filter element itself weighs 1.75 ounces. The cap is a little beefier than the Nalgene cap, so together the cap and filter weigh 2.6 ounces. To offset that weight, if you wish, is to fill your bottle up, then take a nice big swig of water.
This would make it stupid simple to scoop water into your bottle, zap it with a Steripen, and have 100% floatie-free water almost instantly without any hassle. Note: it doesn't work too well on really rigid bottles like the hard-side Nalgene because of the vacuum effect. Water cannot be displaced by air inside the bottle.
http://www.rubbermaid.com/Category/Pages/ProductDetail.aspx?Prod_ID=RP091951
