Help on temperature?

So I have a medical and then drug test at 12Pm. I’ve been told by people in my industry the whole thing takes around an hour and the drug test is the last thing I do, however this is not 100% certain. I tested the quick fix today by heating it up and it took around 90mins to drop from 100degrees to 92. After checking the back of the heat pack it says the average temp is 124F and can reach 156F. This has worried me about if I use the heat pack the sample being too hot and if I don’t use it, it might cool down too much. Can someone offer insight on the use of the heat pack in this situation? I’m using the quick fix 6.3 belt. Thanks for your help.