Need suggestions to power my portable speaker!

Just got done an install, and I am left with a single JBL control 23. I want to use a T-amp and a li-ion battery to power it, so that I can have a mobile speaker with me for various activities. Any suggestion on the T amp? Needs 50 watts single channel at 8ohm. What size battery should I use to get some actually decent use time out of this?

Thanks guys!!