Finding waterproof boxes that meet one's space constraints is tough. Cheaper cases on AliExpress may be of such quality that one fears that opening them would break them. One way going about this is to look into headlamps meant for outdoors. Many of them will have waterproof 2-cell cases for the rear of the headband, even with charging electronics there. As the market for headlamps is wider than for separate cases, it may be easier to find a usable case there without paying much.