Hi Simon
My tanks are all Prestwich. They are 1 piece with a groove in the top for the tuned pipe. Have a look at Dave's website, tanks are on the middle of the opening page.
I use the PT3A but I must admit - the NEWEST is 15 years old
(They still bear prop grooves from when I did multi racing).
They are expensive but dead easy to mount and last forever.
They are not used much in D class as the petrol boats don't use anywhere near as much juice as the glows. My 'A' class uses 1.5 litres in 33 mins (good job the races only last 30 mins)
Most of the D's use the smallish aircraft type tanks, or a few make their own from tin which is soft solder joined . I would have thought your 1 litre would last at least 30 mins.
There was some talk of using IV (blood) bags. Quite a few of the World Champ competitors use them. Very low, and apparently OK for petrol (not methanol, though) :(
For information, here's my 7.5cc one
Good luck
Danny