Of course nobody would expect a math, or English, or any other teacher to write their textbook or other course materials for no compensation. In fact, many districts will pay teachers for curriculum design over the summer. You might try to tap into that somehow (whether through your budget, or maybe curriculum budget). Be aware that they may require you to work during very specific hours, in your office, while your admin are on campus. In a previous district, we had a week of "alignment planning" over the summer with several of the band directors from across the district to plan district-standardized expectations and a curriculum map for band from 6th-12th. We all got paid an extra week (daily rate x5) but had to work together in a conference room at district admin for standard admin hours. They were adamant about accountability with signing in and out daily, and admins popping in throughout the day to check on us.