what if you put fur around your styrofoam sphere? or fur plus a high density foam disk in the middle?
Then you have a totally different concept at work. The idea of the ball is that it is diffractive, not absorptive. I'm not sure that a lump of localised absorption centred so closely to a capsule would be very useful - it certainly won't do what a hard diffraction attachment does - but I guess it's an experiment someone could do. To be predictable, the spheres (and other shaped diffraction attachments need to be hard, reflective and smooth. Non resonant is also good. A critical aspect of these is getting the seal between the capsule and the surface of the ball (or whatever shape) properly airtight or all sorts of nasty sounding and unpredictable things can happen. It's also important that the surface of the diffraction shape is smooth or the diffraction effect varies unpredictably with angle and can produce some strange frequency response and phase anomalies.
Wieslaw Woszczyk's paper detailing his research on different shapes and sizes of Acoustic Pressure Equalizer (as DPA calls them) used to be available from DPA's website but now seems to have been taken down. You should be able to get it from the AES.
Also, Martin Schneider from Neumann wrote a paper, about spheres only, available here:
http://www.neumann.com/download.php?download=lect0043.PDFI've tried using Jecklin and Schneider discs between mics with various shapes and sizes of diffraction attachments and the results generally weren't terribly useful. Sticking with smaller spheres - 30 - 40mm - was variably useful but the larger 50mm attachments didn't really work for me, neither did other shapes. My main problem with them all was to do with changes in the way the stereo imaging worked (or didn't) around the disc.
talk about dense. check out this wooden sphere: http://www.premierwood.com/8-wood-ball-p38419.htm
At 8" in diameter, you're getting into the realms of the stereo 'sphere' mics such as those made by Schoeps Neumann, Josephson and others. These are quite interesting and, in the right circumstances, can produce some extraordinary recordings but they're not by any means a universal tool and do take some getting used to. Ime they don't do what you at first expect them to and it takes some time to get the hang of them but they can be interesting. For me, they're more interesting experimental toys rather than day to day tools but I do have one of these for occasional fun and games:
http://www.schoeps.de/en/products/kfm6.