Never mind difficult it's impossible to find a 29mm-15mm reducer.
Daab, I've just spoken to the pump seller. The 29mm end, has an adapter which the threaded washing machine/dishwasher outlet pipe screws into.
That's why it's an odd size
If you get stuck Frothy for an adapter let me know as I have
a solution.
We use something in fishing called shrink tube. It's also used as labels on food jars, electronics and electrics. If you haven't heard of it b4, here goes.
It's pvc based and comes in various sizes. Basically the tube comes in say a 30mm size. When you apply steam to it, it shrinks to a predetermined smaller size, ie, 15mm. It is flexible before steaming and rigid after