Here is a scanned pdf of my 1984 Radcom article called A Simple Way to Design Narrowband Interdigital Filters .

Doug McArthur VK3UM (SK) wrote a program to implement this design method. Since Doug’s legacy website has now closed, the program is now hosted here. Here  Click to download, then open the zip file and run the install program.

Many people have used this method – design your filter, build it well, and it will work.

A key requirement for good performance is an excellent electrical connection all the way around the base of each resonator element. Use a lathe to countersink the base and make a sharp, level edge all around (there is really no way to do this freehand). Drill and tap a deep hole into the base of each element, and assemble the filter using strong steel screws to pull each element tightly into the supporting bar (you’re actually hoping to make a cold weld).