The subject of on board relief in GA is something that is not often discussed but usually results in a multitude of methods being declared by those who've taken the time to find a solution.
Whilst my Archer can fly for five hours my wife and I can only last two at the most. We are at that age I'm afraid. For me I've tried the tailor made gel bags with a funnel top to be good but unreliable as having to pee 'uphill' into a bag lying at a shallow angle over the seat lip doesn't always guarantee the down flow needed to prevent a flow back. I have some on board as back up but have since found that the 500cl soft drinks bottles fair better in that they'll provide a suitable size inlet with a rigid frame to ensure a down flow. If you see a Lucozade bottle anywhere near my cars or aircraft .........Don't...Just Don't!
Whilst I tried to get my wife to use the gel bags she quickly accused me of not appreciating the female anatomy enough to realise that without squatting getting anything to go where you wanted it to was impossible. As doing so in flight was impossible we've just had to rely on having no caffeine and preferably a bacon sandwich prior to flight to promote dehydration. However, this subject was covered well by a contributor to another forum whose wife has a medical condition requiring quick relief. After much trial and error they found a small Tupperware style container filled with Silica Gel Cat litter provided the ideal solution and catered for the 'variable direction' element of the female relief process.
Hope that helps..... I need to go now............. ;-)