Homebrew waveguide isolator?

[Mrt12]

I am making a waveguide harmonic mixer for my HP 8568B 'boat anchor' spectrum analyzer. First experiments show that it 'sort of' works properly, at least I could measure a signal at 25 GHz... what I am thinking about now is to include an isolator into the mixer to improve the matching of the waveguide input.
However, there is only little literature available on the subject; all I could figure out is that some sort of ferrite material is used, together with permanent magnets.
Has anybody ever thought about making such an isolator?
I do have a working isolator, but I don't want to take it apart to see its internal construction.
Any ideas?