Just put the largest lipo you can fit in your radio box in. Get a y cable to connect anoother one in parrellel and stick it to the side pod or something.
Plenty of room on my chassis for extra lipos. I could probally squeeze 3000mah into the car before things got silly looking.
That way you won't have to stop to switch batteries.
It deosn't have to be pretty. Its only going to be like that for one race.