Did the same just today with my HHKB BT. Used 11 and 8 mm punchers.
Got 0.5 mm thick silicone rings, much better than 1 mm thick dental bands:
Punchers:
http://www.ebay.com/itm/201177951571 ($1.5 for 8 mm and $2.5 for 11mm puncher)
Rubber:
http://www.ebay.com/itm/271225606536 ($2.5 for 0.5 mm A4 sheet)
Feels like an unmodified HHKB but the rattling's gone and it's infact even quieter than dentalbanded. 8mm inner diameter seems a tad too small and you have to recheck if it sits properly or you'd have to reopen the keyboard and realign the rings (I had to), I think it should be about 8.5 or 8.2 mm as here (taken from another post):
Hypersphere rings are precisely 11 x 8.50 x 0.5 mm so they don't sit as tight as 8 mm ones and cause fewer problems.
I'd recommend 8.5 mm inner diameter as on hypersphere rings, but 8 mm puncher (
$1.5) works just fine.
Outer diameter made with 11 mm puncher (
$2.5) seems fine too, there's no need to make it bigger.
Considering A4 silicone sheet (
$2.5) is 210 x 297 mm you could punch at least 210/11 * 297/11 = 513 rings.
Considering
circle packing you could laser cut 210*297*0.9069 / (3.14*5.5**2) ~ 595 rings per sheet.
As for small quantities there's no much need in laser cutting, it takes about 20 minutes to cut 60 rings using punchers.
This method effectively drives the cost to about 1-2 cents per ring.