I had a magnetic oil pan heater for my Rabbit in Nova Scotia in the 1980s - since it didn't have a block heater.  The oil pan heater worked well, but needed to be left on for several hours or overnight.  I can't find the exact one, but here are two similar products (admittedly in Canada):
http://answers.canadiantire.ca/answers/9045/product/0303203P/zerostart-zerostart-super-heat-magnet-questions-answers/questions.htm
http://www.princessauto.com/pal/en/Auxiliary-Heating/300W-25A-Magnetic-Engine-Oil-Heater/4270100.p

James
Ottawa, ON
'91 Westy Weekender

> Date: Thu, 6 Nov 2014 17:28:45 -0800
> From: vwrobb@GMAIL.COM
> Subject: Re: Block heaters for Air Cooled
> To: vanagon@GERRY.VANAGON.COM
> 
> I used a dip stick heater in a Rabbit in Chicago & it worked well, as
> long as the end of the heated dip stick is in the oil it will work.
> 
> Rob
> vwrobb@gmail.com
> 
> 
> At  11/6/2014 12:43 PM, Jamie Fitterer wrote:
> >Anyone know of a block heater for the air cooled van?  This would be
> >used in cold climates for winter starting.
> >