grammar - "Compound nouns list" or "compound noun list"?

I couldn't help but wonder every time I saw such a noun phrase. I've seen both forms used equally often, so I guess both of them can be used interchangeably. But do I guess right?

Some examples:

Here is a frequently used compound nouns list.

Here is a frequently used compound noun list.

I usually rephrase them into something like this.

Here is a reference list of compound nouns.

Answer

All are correct.

But are the compound nouns frequently used, or is the list frequently used?

If it's important to state 'frequently', here are some possible alternatives:

If the compound nouns are frequently used:

Here is a list of frequently used compound nouns.

Here is a list of compound nouns that are frequently used.

If the list is frequently used:

Here is a list of compound nouns, that is frequently used.

Here is a frequently used list of compound nouns. (this one still slightly ambiguous)