The closest hotel is Quality Hotel Nacka. You can find the hotel on the map. “Dieselverkstaden” indicates where the convention is. If you book by contacting the reception at +468 506 160 21 or by email to firstname.lastname@example.org you can use the code Fantastika2020 to get a 10% price reduction. This is not possible if you book via e g booking.com.
Park Inn in Hammarby sjöstad is fairly close to the convention.