Is it permissible to make up missed Friday Dhuhr prayers on days other than Friday, or must they be made up on a Friday?

If you are doubtful about having missed the Friday prayer, the default assumption is that you did not perform it, and thus it is obligatory to make it up as a Dhuhr (noon) prayer, because the original state is that the prayer remains an obligation upon you. You must continue to make up thehr prayer until you are reasonably certain that you have fulfilled the obligation. It can be made up at any time, day or night, to the best of your ability without causing harm.