You really need to have a word with a boiler inspector, especially if intending to run in public.
The cheapest way would be to join a local club, you don't have to be a 'clubby' person although you may become one. That would also take care of the public insurance aspect and they may have workshop facilities that you could use.
Whhatever you do, don't just change the design first, I'm sure that this is an established design that has been 'accepted' as suitable.
Brian