mold release (PVA &/or petroleum jelly)
I'd coat a brush or piece of paper with petroleum jelly and use it to completely coat the inside of the cavity.
Or spray 3 coats PVA mold release into it.
Insert a screw (coated with petroleum jelly) through the hole, seal around the screw with tape or plasticine.
Pour epoxy putty or polyester fiberglass filler putty into the cavity, cure.
Unscrew the screw.
Use a thinner rod in the screw hole to push the plug out if you want to....
If you are worried about the plug getting stuck because of the cavity shape, put a layer of packing tape along the side walls of the cavity before applying mold release. That should let it slip out unless it's a really bad box. Even safer would be to make a separate mold of the fin bases and duplicate them, but PITA, extra steps....
I have used tape over fin holes with good results also, when not wanting to make a little hole into a big project
And I have one board where I filled extra screw holes with silicone caulk, knowing I could dig it out later if needed. Keep in mind silicone interferes with epoxy cure though, if you are thinking of reworking the boxes with epoxy...