I've finally found the time to complete the testing of my silencing rings for Topre-switch keyboards. They have been tested on HHKB Pro 2 and RF 87U keyboards, and (IMO) they work at least as well as the rings used in the Type-S HHKB Pro 2 and the silent version of the RF 87U.
Rings are mode from microcellular polyurethane foam supported with poly(ethylene terephthalate) (PET). Precision laser-cut to the following dimensions:
11 mm OD x 8.50 ID x 0.5 mm thick
Sold only in lots of the following sizes on eBay:
63 (for HHKB Pro 2, 60 + 3 extra), $45.00 + shipping.
69 (for FC660C, 66 + 3 extra), $49.50 + shipping.
90 (for RF87U or Novatouch TKL, 87 + 3 extra), $65.25 + shipping
107 (for RF104U, 104 + 3 extra), $78.00 + shipping
For some reason, the links on GH did not work. Please go to eBay and search for "silencing rings", which should take you to the listings. Otherwise, please try this new link to see if it works:
http://www.ebay.com/sch/katesattic2/m.html?_nkw=&_armrs=1&_ipg=&_from=Rings only. Sliders not included.
Installation: remove sliders from keyboard, slip ring over slider with shiny side up, reinsert slider plus ring into keyboard. Use one ring per switch, including one ring on center of switch for modifier keys.
SOLD "as is". No returns accepted. Will ship to USA, Western Europe, and the UK.
For illustrations, please see the eBay listings.
There is also a thread on DT that has additional information:
http://deskthority.net/help-f53/silencing-rings-for-topre-rf-or-hhkb-t10051.html