Pollen filter for cabin - carbon?

[Whaleshark]

I have been hearing that inside our cars levels of NO2s and other nastys can be found at concentrations of up to 30x what they are outside. 

I have recently replaced my normal pollen filter on my motorway busting passat with a (activated?) carbon one. 

I now want to do the same to my Mk2.   

So, how can I adapt the current leaf grill into a filter housing and then where can I source a carbon filter for it?

[Santiago]

There's an aftermarket kit you can buy that fits in the same way as the original (usually only LHD) option on the Mk2. It doesn't fit perfectly, but it does fit.
It was made by Mahle/Knecht and their part number for the housing that replaces the leaf-grille is LAG19. The filter element itself is their part number LA19.  They turn up on German ebay now and then, but be sure that you're getting the housing, not just the filter element, 'cos the latter is no use to you on its own. Expect to be put off by the prices.

I doubt that an activated carbon version is available, and I'm not at all sure that activated carbon filters have any ability to absorb/adsorb NOx gases, but they do reduce odours, and hydrocarbon fumes, which is always good.
You could probably cut-and-shut an activated carbon filter from a more recent car to fit in the LAG19 housing, with a little ingenuity.

[dragonfly]

Although no longer available - does this help: http://www.partscats.info/volkswagen/en/?i=cat_vag_models&brand=vw&number=1658&set=1676&ein=1991&f=110&hauptgr=1234567890&hg=8&grf=014800014&bf=148&hgug=819&ug=19&parent_id=509226

[Whaleshark]

Both of you, thanks very much.  I have something to go on now.  Have had a look, but can't find anything for sale at the moment.  Will post here if I manage to get something together. 
Interesting also that carbon won't do anything for NOxs.  Well atleast stripping out some HCs has got to be worth it. 

[sparkplug]

I have been hearing that inside our cars levels of NO2s and other nastys can be found at concentrations of up to 30x what they are outside. 

I have recently replaced my normal pollen filter on my motorway busting passat with a (activated?) carbon one. 

I now want to do the same to my Mk2.   

So, how can I adapt the current leaf grill into a filter housing and then where can I source a carbon filter for it?

couldn't you just open a window