VW did, or rather Votex did. Part number 251 091 400
See here:
http://www.t3-infos.de/t3-infos_e.html#innenraumfilter

Might be enough info there for you to make a set?

Alistair

> On Feb 19, 2014, at 6:21 AM, Jarrett Kupcinski <kupcinski@GMAIL.COM> wrote:
> 
> I'm replacing the front heater fan, and so I'm doing a bit of cleaning in the heater box. I was astounded to find a collection of butterfly wings inside the box. I know every van should have a hobby, but I'd rather mine not do this at the expense of cabin air-flow.
> 
> Has anyone rigged up any kind of fresh-air filter on their van? Perhaps something outside, underneath the front grille? 
> 
> Jarrett Kupcinski
> 89 Bostig'd Westy