I leveled mine recently with just a 24" level on the shafts without the props. Front to back. Side to side, etc. Didn't have a large piece of glass handy.
Fwiw, the feet (front to back) were a quarter bubble different verses the tops of the motor shafts. Haven't flown it yet with thee top of motor level to see what happens. I don't know where the IMU sits inside with regards to which leveling system is more accurate, feet or shafts?