Yes, Alaska Seaplanes has a walk in freezer at their facility in the Juneau airport. We normally ship the majority of your catch out a day prior your departure from Pelican (so as to not overload the seaplane on fly out day) and it is held in the Alaska Seaplanes freezer for you.
If you are staying a few extra days in Juneau, Alaska Seaplanes can hold your catch there at the airport for a fee, or you may be able to keep it in the freezer at your hotel as part of your stay. Most hotels in Juneau have freezers but it is good to check how much space they have in advance.
It should also be noted that there is a freezer in Seattle where you can keep fish overnight.