I still use quart sized zip lock baggies. If you catch the urge early enough, there is time to pee and NOT fill the bag. Long flights, most of us should pee a few times, more if it gets cold (read up on cold weather dehydration......).
Part clue is, zip the used bag about 90% closed, thus it empties on the way down. Don't need close to a quart of liquid at terminal velocity hitting the ground or anything else.
I like to try and "go" when up on a street, trim to about 60MPH, then hands off the stick while "taking care of business". If I'm doing a ridge run, pee before getting on tree tops, maybe during a gap jump when it's smoother.
I remember a "well known" pilot saying he used to put a finished apple core in the baggies before filling......sorta interesting.

If I get back to more flying long flights, I shall look at an external catheter, most comp ships I fly have a system in place for that.

Note, if you keep bags in the ship, "air test" them for leaks before filling. Stuff in the little side pockets can wear/punch a hole in the bag. Sucks to find a leaking bag due to liquid leaks......
A small hand towel is also nice to clean any spills.

Women pilots....sheesh, adult diapers are likely the easiest because of their "plumbing" and most glass seating positions.