Hi Ray,
There were one or two people within Gauge 3 and the 2.5" Association who might have done this a few years back but one is (I think) pretty much retired now and the other has (I understand) accepted virtually a full time brief from a commercial organisation. If you PM me – I will let you have their contact details in case they are still interested…
However, building a boiler in 2.5" doesn't involve anything that wouldn't be needed for a larger boiler (and of course 2.5" will be smaller/simpler in most cases) and I believe there are people around who will build in 3.5" & 5" for you – so they should have no problem with a smaller one I would assume – as the methods/materials used are the same.
Regards,
IanT