On one occasion, I held an overheated sample cup in front of a blowing A/C vent in the bathroom ceiling to cool things down.

Partially submerging your sub container in the toilet bowl works too but takes a little more time.

Or try this:

A) Make note of the temperature of your sub container before you pour into the sample cup.
(B) If the sub sample is over 100 dg, re-crotch the sub container, walk out, and tell the collector that you are unable to pee at the moment. The collector will then instruct you to sit down and drink cups of water until you can pee. Play along with the gag so it can buy you some time to allow the sample to cool down naturally. Dont allow too much time for cool down so the temp doesnt fall out of range.

The key to a successful sub is PRACTICE.