They all fall out eventually from vibration wearing away the keeper tabs. You can try Mike's method or mine if you like it.
I've done a bunch by using a #4 or #6 sheet metal screw to "wedge" it in place and keep it there.
Hang the mesh back in it's spot and then screw the sheet metal screw into case at the bottom of the mesh ring at a 45 degree downward angle. This sort of wedges the screen in place.
You don't really want to use glue or silicon because the crankcase vent delivers oil and gas vapors to the box and those will break adhesives down quickly.
If you come up with an ingenious method for doing this chore, let us know.
Oh, and you could buy a new muffler box with a new screen and tabs but way more Cocos and vibration will have that one worn down too eventually.
That is the part im talking about , thanks fellas