Well I arrived home yesterday to a little package on the bench. Yessirree! The Tsunami sound decoder had arrived. Helen was very enthusiastic with a " that's great!" It was 10pm so a visit to the train room was a little late.
Tomorrow has arrived and with a 11am start time I had a chance to see how the decoder would fit. I'd checked and re-checked the measurements and also made up a cardboard mock up of the decoder to make sure there were no mistakes. I didin't count on the decoder not matching to the stated size the manufacturer gives. The issue was the width, and only because the decoder seems to have two pieces that fold back on themselves and are then heat shrinked. The problem is that the two halves are not squarely over the top of each other. This then makes the measurements exactly the same as the widest point of the cylindrical boiler that I wanted to fit it into.
I pulled out the whitemetal files to see what could be done. In a couple of minutes the decoder snuggly fits into the boiler. There is no chance of it shifting, and the contact with the sides should aid in better heat dispersion.
Here's a photo to see how it all fits together, off to work now!