Unless they test right there on the spot, temperature isn't an issue. I have personally put the clean urine in a thin walled plastic container and put it in my sock about an hour prior to my test.
Then I pissed a stream into the toilet and took the container and poured the clean piss into the...
Use an oscillating fan to make the plant sway, that will beef up the main stem. Also, put your plant in a larger pot The bigger the root ball, the larger the stem. Good luck and happy growing.