For my T50RP i also stretched out my first pair of 840 pads so they dont fit. I recommend cutting the tube end off of a pair of dress socks and using that (sock mod) to hold the pads in place. it doesnt really change comfort or sound at all because dress socks are so thin and elastic. If that doesnt work then obviously theres double sided tape or you can make a custom mounting bracket of sorts using some well glued low power magnets, or you can custom design a resin mounting bracket using 2 part epoxy resin and clay. make an impression of the T50RP in the clay, bake it, use that as a "negative" to form a mold and suspend the pads over it.
I tried using a LONG amount of soft tube sock on my ath-m50 as a sock mod. There was just way too much sock. It moved the driver far away, killed all detail, muffled everything, bass went to ****, treble didnt really get THAT affected for sibilance, itjust lost clarity and detail. Needless to say that mod is no longer on the ath-m50. I might try again with half that amount of sock to see if it will help srh-840 pads stick on better but i'm thinking what will work the best is a set of women's black nylons.
I'm still trying to figure out a way to make the stock pleather into a softer material a la how the M50 carrying "pouch" feels like.