You will be fine using that charger! you dont have to buy a expensive charger. that sears charger is a smart charger you can charge AGM, optima, wet cell batts with no problems. I own this one My link
You can put two bolts through the box (one for the pos and one for the neg). and use a single ring connector for each pos and neg. and bolt them using a nut on the out side.
check and make sure your rca wire are not too close to and power wires or electrical harnest. sound dumb but i had the same problem then i rerouted the rca wires and the noise went away