Since you're planning to drain the pool anyway, why even worry about a cover and all the resulting hassle? The walls will collapse without water but you could brace them with 2x4s. If the liner is removed you're not likely to have standing water and a mosquito breeding ground. Then you could build a railing between the deck and the pool to keep people from falling into the hole. It doesn't have to be fancy, it mostly has to be a visual deterrent.