Back up hearing 'aid'

Hopefully, only occasionally my aids need to go in for warranty service - in my case that means shipping at least one of them with over a week turnaround.

Is there a temporary hearing device that is basically the best in class that can be used during the ‘down’ period?

I’ve seen a number of my patients using the Silver Sonic Personal Sound Amplifier and it is well-reviewed on Amazon as well. $25

Good idea!
Eventually you will want to purchase new aids to replace the ones you’re currently wearing.
When you do that, save your old aids for backup in case your current aids needs repair.
I keep my old aids in a large brown pill bottle along with spare tubing for my current aids.

This. I have years of old hearing aids that backup my current.

— Updated —

This. I have years of old hearing aids that backup my current.

Don’t people’s audiologists give them a loaner pair for the time they’re without their regular ones? Mine always does. And she gives me whatever is brand new, so I can see if there’s something I might want to trial for real.

Some do and some don’t.
I’ve used a loaner before. Other times loaners weren’t used by the audi or available.