First, I want to apologize for my POOR english. And also notice that I don't know a word in italian...
I am not sure to be able to make that mod only reading the text. I think it is necessary to view the pictures.
Nicola Mrakovcic, from Croatia, have built with his own tank a bigger one (20 liters), using a metal sheet, welder and dexterity! The result is similar to the original tank, except for its capacity.
Nicola built it following these steps:
1. Cut the upper part of the original tank, that will become the upper part of the new one.
2. Take the sheet ans cut the new part of the tank.
3. Weld the new part to the old tank ones.
4. Paint all.
Nikola has not used some substance to protect the inside part of the tank, but varnishes exist for that purpose.