How can I keep the temperature reasonably safe when I am drip acclimating shrimp

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Hi everyone, this is my first post here. I was recently drip-acclimating some neocaridina shrimp to a new tank. In the process of acclimating over the period of an hour, I had a hard time keeping the water in my other container up at the same level as the tank temperature. I had a little heating pad underneath it but it wasn't really a heating pad meant for ongoing heating so it didn't work that well. Do they make heaters you can put in them? After more than an hour, I was unable to get the temperature up to what was in the tank; it remained always a degree lower.

Does the container matter? Do I need another little aquarium heater or something? Any ideas?

Thanks for all the posts and questions so I can learn along the way, and thanks in advance for any answers.