You need to do the save and quit trick. Position ythe sheild door the way you want it and then, while holding it in place, save and quit. Reload the map and it will have spawned in the plae where you where holding it. Hope that helps
Try pushing it up against the side of a box and then do the save-and-quit-trick. Then you can turn it around to get it how you want it and it is much easier to get it completely horizontal. I used that to make a bridge of shield doors that only vehicles can get across (it looks kinda silly when they're bouncing across it though).
Oops, sorry for the odd formulation of my last post. First turn the shield door in the air and then push the "source/bottom" up against the side of a box. While it's "stuck" to the box you can turn it like any other item. When it's turned the way you like it do the save-and-quit-trick. I haven't tried to move the box afterwards and I haven't tried if it will be "stuck" to the box after you release it without doing the save-and-quit-trick. I could try it and report back later. Edit: Okay, I've checked it out. No the shield door won't mave together with the box after saving and quitting, and no it won't "stick" to the box without saving and quitting. So the box-trick is (as far as I know) only useful for getting your shield door to lie good and flat in the air.
Because either it will fall so that it ends up upside down, or it flips itself and it stands upright.