There are two other possible alternatives, both are carbon fiber and a bit pricey... One is
www.Kiyowatanabe.com, he is in Burbank, Ca. and the other is
www.eti-fuelcel.com, he is is Westlake Village, Ca.
Kiyo is doing a custom rebuild engine for me, and I saw his tanks as well as his other CF parts, all high quality stuff..... His tank is 6+ gals. The Westlake CF tank guy, who lives a few mins from me, make a 5+ gal tank, which he says is 1/2 the weight of a stock tank.... I plan on getting one of these after my engine rebuild is done... If you haven't completed a conversion by the time I get my new CF tank, I will post up some pics and info if you want....