Check out some of these tanks made for marine applications...

Portable tanks:
http://www.westmarine.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=10001&storeId=10001&categoryId=79&langId=-1&subdeptNum=78&storeNum=6

Permanent tanks:
http://www.westmarine.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=10001&storeId=10001&categoryId=257&langId=-1&subdeptNum=78&storeNum=6

They come in all different sizes and shapes, but are mostly on the large side. The portables include a couple of flexible bladder-type tanks that might work well in unusual locations.

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'81 300TD turbo <<sold 12/05 to another diesel nut