There are literally thousands of speakers to choose from on Amazon. What I do is find a good rated speaker on Amazon, then go to Youtube and get real reviews from others who have purchased the same speaker.
From there I can usually narrow it down depending on how much I want to spend.
For my Rad Rover, I wanted something fairly small since it would be mounted on my handlebars. But I wanted to have as much “big sound” that I could get.