For a particular ‘relief mission’ during your visit in Prague, there is now the Museum of Historical Chamber Pots and Toilets, priding itself in the largest collection of items that adequately respond to the human need to go potty. Not surprisingly, the museum shop even sells toilet paper.
Read more about the museum on the pages of Radio Prague.
On a side note, if you want to express in Czech that something is a perfect match (and you don’t mind using gutter language), you can say:
“Sedí to jak prdel na nočník.” – “That fits (literally “sits”) like a butt on a chamber pot.”