Kit: MX644D; IceCube28X18X08-2W
Only minor modifications required to the plastic slope under the coal tender to fit a really good sound to the O4...
Here's another example, almost exactly the same, but with some additional holes drilled through for the sound to escape from.
Photos provided courtesy of Mark Larder and Stephen Payne
Kit: MX644D; IceCube28X18X08-2W; LifeLink+4x Tall Tantalums
Same approach, but finding a neater location for a more compact stay-alive - LifeLink PCB with a small array of Tall/Narrow
Tantalums joined in parallel, which tuck neatly under the decoder.