prep about 1.5 l of porridge/oatmeal 1 day in advance.
wear a diaper (2 if you are worried
) and some tight trousers.
drink 1.5 l of water and then take the train to the other side of the city. Find a public toilet and fill up your diaper!
you can go home now.
if this is too public. you can do the same thing but sleep in it instead
you can go home now.