If it's your own aircraft, or one you use very regularly, then perhaps you could fit a 'Pee Tube'. These tubes are available from most good aviation 'chandleries'. They consist of a 'Pee Cup' with a legnth of pipe that goes to a venturi under the aircraft and in the prop slipstream. The suck they produce is remakable (I will leave the obvious comment!) and they can be used equally well by women.
I once fitted the device for a friend of mine who had an arial photography job and spent so much time aloft he had to have surgery for kidney stones!
His airborne life improved imeasurably. Further, we were able to tuck the unit out of sight behind part of the carpet that went up the side just behind the rudder peddles. - All certified and approved!
If you own a Cherokee Arrow, don't pee into a bag and put it out of the DV window. I know someone who did that in Africa, it went over the pitot head on the aircraft side, the one that controls the Gear.
"What - no airspeed? - he must be landing". So the gear went down and the resultant drag meant he could no longer make his destination, and in Africa that's bad!