I have used an all stainless ring clamp to secure the herb/tea ball to the pressure relief valve nub on the underside of the lid. worked great. the 1 oz of pellets did swell up pretty fat, so I would not use more than that, but I didn't notice any sediment escaping.
Also, the herb ball chain is not too long, so the ball will only be submerged in a full 5 gallons. I suspect that after a few pints it will be hanging above the beer. Of course, if you cold crash or carb for 1-2 weeks prior to drinking, that's perfect, since you won't get any grassy notes from overextending the dry hop contact time.