Decided on the Defi boost gauge with Steve's adaptor.
Here is a link on the adaptor and more on taking the vent apart:
http://public.fotki.com/ttschwing/gadgetts_and_stuff/52mm_vent_gauge_adapter/

Here is another good thread on installation by BlueTTop.
http://forums.audiworld.com/tt/msgs/1008458.phtml