I have just a little question concerning composita. When I want to
make a compositum I never know whether to use the genitive plural or
singular for the first word. For example (a quiet funny example) the
word powerhead. Would it be "alfskjannr" or would it be alfakjannr"
or should I use the nominative instead, so "alfkjannr"?

And another question at the end: what about a word like dygð? Is the
genitive plural dygðs, as I looked it up as an a-declension or would
it be something different, because it sounds quite weird?

P.S.: I know, I do not have much knowledge about Old Norse, so those
questions are a little bit stupid.