@Gryffin,
Not an expert, but if you want to say it's a panel composed
of judges, it would be "judges panel".
If you want to say it's a panel which other judges have to recognize is the judge of
them in regards to something, it's "judges' panel".
If it's some sort of situation where this panel represents the profession of judges in some way, as opposed to "doctors' panel" or "plumbers' panel", it's "judges' panel".