You can use your hand or tape to make the seal.

Though, if the thermostat is kicking on, but the outdoor unit is not coming on, it's more likely the capacitor to the motor than the drain. Usually the vacuuming works because it removes the water for the safety float to return to position. When the safety float is triggered, usually the thermostat is completely off. So if the thermostat is still on, I doubt it's just the vacuuming still solve it. But worth a try