Generally we use twist locks. [Linked Image]

This looks to me like a under a data room floor.

If it is under 645 rules the liquid-tight need not be secured, the same for the box. 645.6(E)

We do a lot of installs where the UPS maker supplies pre-made liquid-tight whips with FS type boxes and receptacles.

For better or worse all you do is drop them in like extension cords.

Bob

Bob Badger
Construction & Maintenance Electrician
Massachusetts