I have two Amp Caddy Pro speakers on one cart and two Amp caddy Max speakers on the other cart. They work great, clamp on to the roof supports easily, are water proof and seldom need recharge. I had a minor issue with two and the company replaced them completely - excellent customer service. The sound is excellent and you can synch two speakers in one cart if you desire. I highly recommend them - they can be purchased on Amazon - just search Amp Caddy Speakers. I also purchased their phone holder and it also works great.
Amazon.com