What Dogman said and the best place to find cool bags is at a thrift store.dogman dan said:One option you might look into is bags not intended for bikes. Tool bags for one, and also old camera bags from when video cameras were gigantic are some of the options out there.
Very hard to find bike bags longer than 12-13 inches. (30 cm or more) Lots of batteries are unfortunately 14 or 15 inches long.
But in the bike bags, the topeak bag and rack is one of the longer ones.
torque plates for a rear motor that also include a much beefier rack attachment point.