@S-Kippy no need for hot water or freezing to create plump dead maggots.
Two ways you can make them. Firstly put the maggots in an air tight container overnight like the Fox Buckets with lid or smaller such containers.
Maggots go dormant from lack of air and when at the bank transfer the amount you want straight away into a container of water. This will kill them.
With the lead off, the other maggots will come back to life again for you to use.
Dr Paul Garner talks about storing & flavouring maggots on the bank in his video.
The second method is similar and in Jamie Harrison's video.