If we suppose the "3 free slots" popup is clientside, then, if we remove it, we'll be able to use the moonbox, but because the stuff you get from it is serverside, it won't use the moonbox serverside (3 empty slots error).
In this case, there is 2 solutions:
1->if there's 2 or 3 items in the MB, it won't open at all; and if there's 1 item it will. (what we hope)
2->if there's 2 or 3 items in the MB, it won't open at all; and if there's 1 item it won't also, because of a string which tell the server, even if we release one item, you MUST have 3 free slots; or each time we open a MB the server may generate a soc stuff/elite stuff AND 2 stuffs counted as empty. (worse for us)
In other words, are the free slots serverside-checked also or not?