You need an oil pan heater (I have one) but I can't find one online.  VW dealers here in Canada used to stock them but not sure who made them.  Canadian Tire has something like this:
http://www.canadiantire.ca/en/pdp/polar-pad-engine-heater-250w-0300001p.html#.VFwG0IcTZUQ
No experience with a stick on type.  Back when I used to run my vans year round (into -30 weather) I would use an oil pan heater and leave the trickle charger on all the time.  Had mixed success with a battery blanket.
Jeff

> Date: Thu, 6 Nov 2014 13:43:46 -0700
> From: fittdog8848@HOTMAIL.COM
> Subject: Block heaters for Air Cooled
> To: vanagon@GERRY.VANAGON.COM
> 
> Anyone know of a block heater for the air cooled van?  This would be used in cold climates for winter starting.
>