Can I ask which considerations did you base those measurements on?

Using the prescription from

http://gregegan.customer.netspace.net.au/SCIENCE/Cavity/Cavity.html

I found the following resonant modes:

TM010: 9.65 GHz

TM011: 14.97 GHz

TM012: 21.07 GHz

TE010: 17.05 GHz

TE011: 21.02 GHz

TE012: 25.24 GHz

which do not seem to correspond to the 24 GHz source you mentioned. Is the cavity optimized on a different mode, or on something else entirely perhaps?

I wrote a mathematica notebook to calculate the above modes, and I'd be happy to share that with you. Keep up the good work!