well i have done this before...but i used a water bottle...that was wrapped in plastic MANY times then marked in my freezer... it works great if u can substitute.... i am supervised so its no good for me now...one piece of advice i will give is that if u are freezing then thawing piss...make SURE u shake it A LOT after u thaw it...or else there will be a grainy look to yer pee and the tester will know it has been frozen and thawed....as long as u shake it alot u will be fine though... also dont forget to make sure temps are right!!!!

edit: i will also say that i used the SAME pee over a 1 year period and came up clean EVERY TIME... never had problems...i would like to know the shelf lifef of pee in a freezer though...thats some good info to have on hand... i personally know mine was good for up to a year after i froze it...