Just wanted to let you all know my Grandma loved her gift of podcasts! I opted not to go with the Amazon echo after talking it over with some family - she was never technologically inclined and is even worse now in her old age, and doesn't have the patience to learn to use new things. I wanted this to be something she could use on her own, so I went with USBs on a key ring and tactile labels. Plugging those into her audiobook player would be more similar to using a cassette tape or CD, which she is very familiar with.
Funnily enough, her favorite part is that I made recordings of myself giving the title and description as the first track on each drive - they all start with "Hi, Grandma!" Thanks to everyone who helped me out!