Which direction are you trying to dump? Into or out-of the can?
The scatter going inward is being deflected rather than trapped. It is still attenutated which is better. That which is reflected off the cavities/window is normally reflected back off the black (green) glass. Would we be better with just two pieces of glass (in a Vee)? Or a double trap with a Vee pointing forward and backward (an X-dump?)
Scattering is a huge problem in our setup, and we aren't sure where exactly the offending scattering is coming from. The most basic thing to do is to go through our entire optics table, find all stray beams, dump them, then see what kind of spectrum we're left with. At that point we can try more advanced techniques, like buzzing and damping resonant optics or upconverting the scatter source out of our band.
Many stray beams are coming directly from our cavities and polluting our transmission table. Also, some beams are trying to make their way back into the can. These beams tend to be close to the main beam, making dumping difficult.
To aid with dumping these mutant beams, I have created what I call the Sentinels. The Sentinels stand guard at the transmission window of the vaccan, daring any puny beams to interfere with the main beam.