How to transcribe a spoken abbreviation in writing

^{(This question was inspired by comments in a similar – but not duplicate – question.)}

Suppose I'm tasked with transcribing a physics lecture, and the instructor says that the answer is
“42 km/s.”

In this case, it's important to note that the instructor did not say the words, “kilometers per second”, but instead said (phonetically), “kay em per ess”. As the transciber, I want to somehow capture this subtle distinction – that is, I want it clear that the letters were spoken, not the words.

I can think of a few possible ways to do this. One is to spell out the letters phonetically:

“...and that gives us our answer of 42 kay em per ess” ¹

Advantage: How the answer was enunciated is clearly communicated.
Disadvantage: The resulting form is ugly as muck.

Another option is:

“...and that gives us our answer of 42 km per s”

Advantage: It's easy to read and pleasing on the eye.
Disadvantage: It's ambiguous; the reader might assume the lecturer said “kilometers”, but I've taken the liberty to abbreviate that as "km".

I think I might prefer this third option:

“...and that gives us our answer of 42 k-m per s”

The hyphen might clue the reader in that the letters were spoken, and it would prevent us from having to write out really ugly phonetics, such as “em pee aitch” for MPH.

I suppose the most practical answer would be to include some clarifying remarks in a preface to the transcript that explains the conventions that are used – that is, something along the lines of:

When abbreviations are used, those are denoted with a period (e.g., “mm.” for “millimeter”). If the period is omitted, you can assume the speaker pronounced the abbreviation letter for letter (e.g., “mm” was pronounced as “em - em”)

Here's my question, though: Are there any standard ways to handle this problem?

As a footnote, this problem seems particularly vexing when you consider how plurals might be used. For example, a lecturer might pronounce three meters as “3 ms” (phonetically, “3 ems”), but the abbreviation might look like three milliseconds. Even though apostrophes in conjunction with abbreviations are falling out of favor, I suppose this might be an exception. According to the accepted answer here, it seems like this may be an acceptable use of an apostrophe (3 m's).

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¹ _{(Let's assume we are allowed to write numerals and numbers, and do not have to write the answer out as forty-two.)}