This design was created from scratch to cover the hole a typical HPA conversion leaves on the back of your upper receiver. Sure, you can still put the original charging handle (with spring) there but... come on, we want a lightweight toy and not a fully featured replica. Some people designed somthing similiar but charge you money for it, which I do not like for such silly and easy things. It took me like three hours and two prints to get it good enough for my needs.
There is some slot where the gearbox should fit in. I was not able to check / test this because my own gearbox is cut down at the back.

The design itself should be press-fit. I used a "heavy beating device" (hammer, piece of wood, whatever fits the bill) to insert the cover for the first time. It is specifically designed for Specna Arms Edge 2.0 metal uppers from the E-series.
I don't know if it will fit any other replicas but you might give it a try. Printing time is about 30 minutes with like 3g filament.

This will be part of a stock cover (to hide the ugly ununsed part at the end of the lower receiver) which I am currently developing.
As usual I included the SketchUp file in case you want to modify anything. Please note that the surfaces are not really equal flat and straight.