I did my research and ended up getting a UE Megaboom. Can be had refurbished on Amazon for about $200. Battery lasts 12+ hours at 60-70% volume, which is more than enough. Fits in the cupholders. Waterproof, just in case. Uses bluetooth or auxillary in.
The drivers who have heard mine have been impressed by it, especially when the notice I leave the building with it on and show up past 2000 some days and it's still going, even a few customers.