When I was a young guy taking flight instruction(almost 50 yrs ago) the Skippy Peanut Butter jar was the prefered "relief tube" for X-country flights 
Wide mouth, good sealing lid etc.
EdH
: This was covered in the Texas Kayak Fisherman forum, (link below) Interesting
: problem, depends on gender etc. but there is always a plastic bottle for
: that purpose in my yak. Use it, rinse it, and put the lid back on.
: Tony
