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Evanescent filters (meaning in waveguide below its cutoff frequency) are a useful way to get reasonably narrow band high-Q bandpass units. Here are some designs for 3.4, 5.76 and 10GHz all using short lengths of standard waveguide sizes. File:EVANFILT.pdf

AADE Filter design is a pretty good package for lumped element designs. Available from here :

Dom F1FRV has an excellent design sheet for coaxial stepped element low pass filters including end corrections. The site is in French, but the spreadsheet is easy to follow.