Hello, I've been considering this product:

https://www.amazon.com/Waterproof-Bluetooth-Portable-Wireless-Playtime/dp/B09MFBNTG3/ref=sr_1_8?keywords=portable+stereo+speaker&qid=1657368918&sr=8-8

Stereo, with Aux in, and battery powered - It's no monitor but would probably work for busking.