grammatical number - Pluralization of acronyms

I have seen the following questions: Pluralization of acronyms ending in 'S', What is the correct way to pluralize an acronym? or Plurals of acronyms, letters, numbers — use an apostrophe or not?

In all of them, the plural of in the extended form goes in the end:

One compact disk, two compact disks

One automated teller machines, two automated teller machines

My question is, what about acronyms (or initialisms...) such as

One rotation per hour: 1 RPH

Two rotations per hour: 2 ???

(Note that RPH is a home-made acronym.)
Obviously RsPH is not an appropriate choice, but RPHs seems a bit strange (I would be fine for rotation per hours, but maybe not for rotations per hour).

Any piece of advice?