Peter,
friend of mine who used to do ferry flying told me his standard reply to the kind souls who greet him at refuelling airports with "Welcome to .... " was "Thank you, where is the toilet?".
I did read a great account of a German fellow who flew a motor glider from Germany to Australia. He had a small bag with him for use when he needed relief which he would then empty out of the storm window. It took a few tries until he found which end to push through first in order not to generate a spray can necessitating a thorough shower as first order of business after landing.
Those bottles are good solutions, some of them even come with rather cleverly designed bags to carry them in, which make them look like camera bags or handbags for the Jane variant.
Best regards
AN2 Driver