Talk to cyclists... there's plenty of ways
one method... and here comes a fun procedure
before the piss check, they will insert a tube into their urethra
all the way up to the bladder, tickle the bladder valve (dunno the word in english for it)a bit, so it opens, drain the bladder completely, then they will inject other, safe urine in there... and then it's off to the piss test
great fun, innit?
off course, in less difficult enviroments, one might simply stick a bag under the arm, with a tube down low, and squeeze the arm to piss manually and clean... but it won't work if they are watching you closely...