Dave,
Contact Ronco Plastics and arrange to fax them a drawing of what you
want and they'll build it for you. You may remember a post of mine a few
weeks back where I described having two different tanks made by them for me
for two different sailboats we owned. In both cases they were smack on to
the size I needed and the price was very fair.
Go to their web site at http://www.ronco-plastics.com or call them
directly at 714.259.1385 to discuss your special needs. Note that both
times I dealt with them, the price I paid was 50% of the ones listed in
their on line Catalog. That also included all the fittings I needed thrown
in for free. I even took advantage of their offer to add a 3" inspection
plate (with O-ring sealing threaded cap) on the top for future cleaning with
a garden hose. That probably is not necessary with the heads we use on our
Airstreams but I thought you'd like to know that anyhow.
Good luck,
Glyn Judson
1969 Caravel
Santa Monica CA