There are some useful papers about this problem.
I don't have theirs. May be anyone can upload this material:
A simplified analysis for high power microwave bandpass filter structures
Sivadas, A.; Ming Yu; Cameron, R.;
Microwave Symposium Digest., 2000 IEEE MTT-S International , Volume: 3 , 11-16 June 2000
Pages:1771 - 1774 vol.3
Analysis of power handling capacity of band pass filters
Chi Wang; Zaki, K.A.;
Microwave Symposium Digest, 2001 IEEE MTT-S International , Volume: 3 , 20-25 May 2001
Pages:1611 - 1614 vol.3
Prediction of peak internal fields in direct-coupled-cavity filters
Ernst, C.; Postoyalko, V.;
Microwave Theory and Techniques, IEEE Transactions on , Volume: 51 , Issue: 1 , Jan. 2003
Pages:64 - 73