stored piss bags always works for me.
get clean, and stay clean for a week or so. piss in a baggy. a big one. fill it up. store it in the fridge.
on piss day, poor some in a smaller baggy (the tiny kind). put a straw onto a corner of the bag and seal it all up. got glue works well. clamp the straw shut.
stuff the baggy of clean piss under your scrotum with the straw facing your zipper. when you go to pee in the cup, unclamp the straw and "pee" into the cup. done.
i don't condone this...
~Andrew