from my personal experiences, the ofna chrome top/blue bottom boxes work really good for the price. I just sold one with batt/charger/power panel for $50 used...worked perfect. Those boxes are only like $65 new (minus all the extras) and usually can be found for really cheap used.
I bought one last year at Leisure's old track for $25 with batt/charger. Plus they have a huge motor and will turn over anything. WAY stronger than two 550's