Somebody must make them for felder and almost certainly in China.
I read of helical block users finding cheap sources by matching dimensions, angles and radii but no specifics
Is there not a felder user group?
A bit of research gives 13.8mm sq 2.5mm thick and 150mm radius for the silent power heads.
They seem to sell for £25 for 10 so dont you just have to accept that as running costs for that class of machine?
I'll not make it too easy for you and provide a link. Be good practise for your interweb skills
Bob