The left cover shell on my new King Song KS18XL came loose after a couple weeks at around 500km. When it happened, I took off the cover shell and found that it is screwed to the body through nuts that are semi-embedded into the body and glued.
My first attempt at repairing was to super-glue all of the nuts back on. It held for another 100km, but pulled apart again, and then the right side shell cover also started having the same issue.
Knowing that re-gluing was only a temporary fix, I was determined to come up with a permanent solution. I decided that backing screws would be the strongest, but the original screws were too long to allow a second screw through the other side.
I recognized that the screws are the same as used in computers for securing a motherboard to the chassis, so I started searching through the screws I had accumulated over the years. I found that I had the correct length screws and enough of them for the entire project. The replacement screw lengths are 6mm for the cover shell side (down from the original 7.67mm), and 4.75mm for the inside backing screw.
I used a rounded head for the inside backing screws, but ideally the screw head would be flat and thin. The third of the five screws has a possibility of rubbing the tire. However, I've found that with the improved rigidity after the upgrade, there is no rubbing. Before the upgrade with the cover shells separating, the body flexed enough for the tire to rub against the inner side of the plastic body.
I found many videos on YouTube with KS-18L disassembly instructions. I liked KingSong KS-18L tyre and inner tube change - disassembly - Justina's Garage (ep. 6). I figured that removing the wheel from the body would give me enough room to install the backing screws and I was right, but it was difficult and I needed a special tool. I used this PITTSBURGH Right Angle Screwdriver Set from Harbor Freight. Taping the screws to the bit helped a lot for the hard to reach holes.
I hope that King Song incorporates this improvement into their design. I also highly recommend that anyone with a KS-18L or KS-18XL do this upgrade. Especially since I and rode with with the cover loose and lost one of the nuts to vibration.