i got one to suggest .... its from ATL... it fit perfectly under our spare tyre space... i will fill 12 Gallon (40L)... the diameter is 22'....i found this setup been used by a racer from japan... nice 1302...
the price im not sure around $536.00... and they have an 8 Gallon one for $498.20....
more info go to...
www.atlinc.com
hope this can help u abit in ur journey..