Adapter to fit a standard Plano Field Box to the back of the Engwe M20. It's not a ton of storage space, but I found it useful enough. It also helps keep mud and dirt off your back, as the stock fenders don't do the best job by themselves. 

Uses 4x M6 x 20mm bolts and nuts to attach the box, and 5x heavy duty zip ties to wrap around the frame. Should fit other similar sized field boxes. 

Field Box: https://amzn.to/4gXiflh