For my Syncro, the ribbed, black plastic hose that connects
the snorkel to the dust trap elbow is listed in the ETKA
as 60/62 mm by 712 mm.  Part 251 129 615A.  Still available
from the Bus Depot for $105.

That size is almost exactly 2.5".  I have on order some
ducting for heater/air conditioning use in that size.
http://www.nostalgicairparts.com/air-conditioning/2-5-inch-flexible-duct-hose-57.php

The snorkel is the same part (251 129 534A) between 2WD and
Syncro.  The 2wd hose is not identified as to size, so it may
be different from the Syncro.

Not shown on either page is an adaptor from the oval snorkel
to round hose, which I have.  I should have that 2.5" hose
tomorrow according to the USPS site.  I'll report on whether
it fits, or how well it fits.

Richard