Hi, I don't know about your circuit, but the idea to avoid propagation of umwanted waveguide mode is to keep the cut-off waveguide mode (TE01) well above the operating frequency of your circuit. For microwave housing design, usually the circuits (such as filter, amplifiers) are contained in narrow channels to keep the waveguide mode frequency at least 3 times higher than your operating frequency. For example, for 15GHz edge coupled microstrip filter etched on ceramic hardboard, we keep the side wall of the filter housing just about 5-6mm, height.
You may search triquint 's milimeterwave IC application " Design Guidelines for microwave cavities" for detailed discussion.
Alternatively, the Artech House book " Microwave Component Mechanics" should be able to address your problem.
My point is: Don't rely on microwave absorber. I have reverse engineered so many commercial microwave components. Hardly seen ones which use absorber. 8)