Most of the time the hot tub cover is present & I will use it. I use 1x4's to "sandwich" the flaps on all 4 sides, then install hasps on the 1x4's and A389 all 4 sides.
If the cover is missing, use plywood with 2x4's screwed on edge on all 4 edges then hasp & padlock.
If neither of those methods works, or there is an obviously easier way, I will improvise. Seems like there are lots of right ways to secure hot tubs. I've used multiple methods and never had anyone ask me to return and redo any of them.