You are a little unclear with " this is my spare . . . " , is the motor in the car?

The part may or may not have fallen in.

First, have a look with a flashlight at all of the oil drain holes, it might not have made it to the bottom. Use a magnet to pull out as grabbing it may cause it to fall further.

If motor is out of car, turn it upside down and shake. Pulling the pan is an option as well. ( Some cars have a steel lower pan that is easy to pull even when in the car. )

Is this cap larger than the oil drain plug? If not, a magnetic plug would eventually grab it.

The cap could get caught between a connecting rod and side of crank on the way down but things would have to be just right for this to happen. I'd turn the motor by hand at first. Once the cap makes its way in to the pan, it will just sit there. The oil pickup has a screen on it to prevent pieces from being sucked into the oil pump.