Nevertheless the HD800 pads from Dekoni fit my 820 just fine. They indeed are much thicker than stock 800 pads, but as I recall about as thick as the 820 pads so probably wouldn’t help with fit. I believe Sennheiser at some point actually suggested using a rubber band to clamp them together when not in use so that they fit more snugly. I agree that they really should have more clamp force and that this has really harmed the reviews they get. Some get little bass and describe them as honky without a good seal. I do see they mention only the 800 and not 800S, but I thought the pads on those two were the same.800S and 820 pads are different and not interchangeable. The 800S pads are much thinner. If anything the 820 needs thicker pads. Or some other modification/trick to increase clamping force. Best I've been able to come up with is a giant rubber band wrapped around my head lol...
https://drop.com/buy/dekoni-pads-for-sennheiser-hd6xx-hd700-hd800#overview