I bought one of these a while ago and it works great:
https://www.amazon.com/Portable-Wate...-1-spons&psc=1
• 360 degree sound with 8 symmetrically placed drivers
• 100% waterproof
• Fully dustproof, mud-proof and shock proof.
• 35-hour battery life
• Portable Power Bank - Quick charge your phones and tablets
• SIRI & GOOGLE NOW control
• Built-in full duplex speakerphone for hands free chatting.
It can be VERY loud with little to no distortion. I have it bolted down in between the front and rear seats so it is pretty hidden from view but still sounds great.