Behold: FOUND!
That part is really great. Finally relaxed lie down without being worried about the stupid screen.
Simply open the 2 levers on the left and right and in turn like a screw; Then insert the screen and with the red ring clamp the screen. That's it!
The whole is easy and holds bombproof. I have the ground socket used on soft and hard ground, for use on gravel, I can not say anything.
Conclusion: If you want to place a relaxed screen for the ground sleeve is a MUST!