*Membership spots not really limited!
There is a sidequest for Spec Ops 6 which give lockboxes as a reward. It seemed to open up when I got through the "Shattering Punch" task.
If you go through all 6 side quest tasks, you end up with 80 lockboxes.
Of course the last one is 4 star mastery on all three chapters
ive already 4 starred the first one