In the case that you are missing a felt pad entirely (just a bare cup), you can still fix your flute temporarily.

Cut a small strip of teflon tape (about 4" )

Holding both ends, slip the tape under the key with the missing felt pad. It's quite the surgery, but manageable.

Make sure the part where the key makes contact with the body is fully covered.

With the remaining tape, twist it together at the top to hold it in place.

This is a temporary fix, but works perfectly.