As per the information I have, there is no summon portal tied to the Monster Island event (at least for now).
So for the Abyss Hunter heroes, Otis and Goretooth will be featured at the next Covenant summon event (EDIT: on the 9th Dec). Bogart and Basil will also be part of the portal’s pool. For the other Abyss Hunter heroes, they do not have a released date yet.
Hope this clear things up