Hi Neil - I think you might have to go up to Z Pro filters and holders for 82mm. My 77mm adapter looks like it is pretty much at the limit for P type holders and filters. I would imagine that Cokin P holders would vignette on an 82 mm lens.
You can get third party equivalents of the Cokin A and P holders on Ebay for 2 or 3 GBP. There are just plastic and so there is not much to worry about in terms of quality from the Cokin original. I'm sure the Z Pro size will have some third party versions as well. I also buy cheaper adapters from eBay - as long as the screw thread is ok there is not much point to buying an expnsive one.
What lens are you using with a 82mm thread? Sounds like a monster
Nex 5, Nex 6 (IR), A7M2, A99 and a bunch of lenses.