kristofa:
They are overlong (16" W X 30" L) bases with a 1/8" plywood support (16" W X 32" H) screwed at right angles to the back end of the base. On the front of this right angle (speaker facing), one sheet of 1" chair foam batting and two sheets of 2" EcoTouch home insulation are tack glued. Then the back piece/foam is covered in speaker cloth, tack glued and stapled. Once you have the materials and basic idea, takes an hour or two to build.
AND, regardless of the simple design, THEY WORK. The DI cabinet pressure is not affected but the 180 degree out-of-phase port noise is killed. Will very lightly reduce the total bass volume but will substantially improve accuracy and presence. I use REW EQ to bring the bass volume back exactly where I like it -- about 5db high from 16-80 hz.
BTW, have also gone over to the half grill alteration discussed towards the end of the epic DI/Corelli thread. Have used this for two weeks, retuned my room response, and will not be going back to a full grill. Check it out.