I use a Coghlan's fuel filter with the mesh removed. It was $1.97 at Walmart. I hike in a running skirt that has the shorts built in. It's easy to pull the leg over and use the funnel. I've used it for several weeks and haven't had any problems at all.
If you want a laugh, you should read our (my hubby & I tag team a blog) on this very subject at thedirtyduo.com. It's been tough for him giving up the "I'm the only one who stands to pee" status. He's adorable.