I use NATO-style cans both in the luggage rack above the driver (lashed down well) or standing up in front of the pantry door and behind the drivers seat, lashed to the table leg in it's mount ( a Westy application, obviously). 

I have watched for but have never observed the swelling mentioned in the original post, since I  carrying diesel, but I would definitely watch for
It. Put your boat container in the sun with an insulator of some kind on it and see what happens. And remember those containers were made to sit out in the sun in a boat. 

Jim

On Aug 14, 2011, at 6:38 PM, John Rodgers <inua@CHARTER.NET> wrote:

> BMW makes (made?) a spare gas can that fits into the space in the
> spare tire. Turns out that this gas can fits exactly into the space in
> the Vanagon spare tire.
> 
> Look for BMW Spare Gas Can on E-bay.
> 
> Maybe not what you want, maybe not the price  you want, maybe not the
> convenience (inconvenience - under the van the spare wheel carrier)  -
> but it fits, it works.
> 
> I have one of the metal ones. Works great.
> 
> John
> 
> John Rodgers
> Clayartist and Moldmaker
> 88'GL VW Bus Driver
> Chelsea, AL
> Http://www.moldhaus.com
> 
> 
> On 8/14/2011 1:10 PM, Roger Barkley wrote:
>> Am trying to come up with a way to carry some spare gas for an upcoming
>> sojourn. Thought an outboard boat tank, (one of the flat ones), strapped to
>> the roof cargo area might be a good container untill I purchased one,
>> (Attwood). The thing swelled in the sun, made me a little nervous. What have
>> others done, used ?  ('84 Westy).  Thanks
>> 
>>