Thank you everyone for the input. I ended up going with a Samsung 64G drive because it was same-day delivery in my area and paid a little extra for the low-profile model:

https://www.amazon.com/dp/B07D7P4SY4

The Unraid boot device creator was happy with it, unlike the SanDisk 32G UltraFit from Amazon that was my first attempt.

Off-topic musing: Wouldn't it be easier to have the option of creating a small boot partition of the required size on an SSD? The rest of it could still be used for caching/performance.