You want the B+W XS-Pro Kaesemann MRC Nano Filter in 82 mm. It's relatively slimline but still has filter threads on its front so you can use a lens cap or in conjunction with a Lee or Cokin filter holder (I prefer this approach to the Lee 105mm circular filter mounted to the front of the Lee and that costs even more - the B+W is better glass). The 'slim' non-XS-Pro variant doesn't have filter threads. Stupidly expensive in 82mm and you'll have to get onto B&H's waiting list to get one - in 82mm, they're still hard to come by. XS-Pro is B+W's top of the range filter; with this lens you should be using the best polarizer available. This is not one to compromise with.
With 24mm (on full frame), you're just about OK with a polarizer. Any wider (e.g. Zeiss 21mm), you'll have to contend with uneven skies. Vignetting will be an issue with extreme shift with or without a filter, but you won't have filter shadow if you keep the shift within reason with this one. When you get used to this lens, you'll find you don't need to use too much shift (or tilt) to get what you want. Less is more. In common with most B+W polarizers, I lose between 1 & 1.5 stops or so depending on strength of effect (i.e. rotation). Do play with the lens before you go away with it - it's not a beginner's lens. Be careful not to tighten the lock knobs too much. I don't tighten them at all. They're easily broken and expensive to fix.
If you stump up for this filter, you can at least use it on smaller diameter lenses with an appropriate adapter ring. Seems like 82 is the way of the future. It's a bitter pill, but you don't have to take it often. Hope this helps.