First, check with the vendor as there may be an accessory bag available. Is the 18 inch size across or the height? Can they fit in the bag on end? It may be hard to find the exact pattern size you need. Perhaps draw a pattern on paper and add in one inch for extra allowance. Add some Velcro tabs on the inside to secure the batteries in place.
If the two can be carried together, something like a wine bag with the separate sides might work also. The plastic grid canvas can be cut to size for added stability in the bag. Good luck!