Possible solutions:
ZipLok bag ("glider pilot's friend")
Nalgene wide mouth jar. (!)
"Little John", which has an available "Lady J" adaptor.
We all like to make a joke of this, but even in a lowly 172, having to land the airplane when you are really full down there can lead to disastrous results. It can only be worse in a complex airplane. (My choice was the second above!)
Meanwhile, go get a urinalysis and culture. Bladder infection = no big deal. However, untreated can lead to kidney infection, which indeed = big deal.