One of those pockets that are used when a DIN slot isn't used might be good concealment... The pocket used in my Polo fits a MkII sled perfectly (a MkI is a bit tighter fit) if one shaves off all the extra bits and pieces outside the basic pocket.
See http://www.tempsch.org/empeg/misc_empeg.html (it's on an ADSL line that has been slightly shaky lately, so try again later if down)

IIRC someone tried to look up that part# at a US VW dealer, but had no luck finding it. Might be that the Polo isn't sold in the US...

/Michael