New ideas at 6/10:
1. Restraint chair maybe a good direction or somethings similar. Sit tight and hold tight.
2. Similar to the first point above. A ideas coming from a doujinshi, the name which is restraint boxes(close in meaning), the content of the book is mainly about to lock the girls inside the locker, and do "somethings" what you want. I suggest refer to the content of the book, content is appropriate.
3. Similar to the second point above. This idea may enhancing the passive locker in normal, a big option.
At last, a strong suggestion, hopefully this game can be selling in Steam platform. Steam is allow the game with adult content, also selling the game in Steam can be more easier to promote.