It wasn't as complicated as it is going to sound....
A circle of larger diameter of the centre section hole, where the paddle wheel will fit, ( same thickness) then scribed a hole the same size as the main hole, in the middle section.
I set up a drill bit in a drill stand ( around 5/16 th from memory)
I "pinned" the axel to a timber support, and drilled holes so that the centre of the hole, aligned with the scribe mark, about 2 mm apart.
I calclated the exact distance of drill dia and spaced the exact distance in between the holes. ( sound confusing

) basically hole, space, hole , space etc and when calculated, there is the correct distance for the last hole and space .
Rotate the disk on the pin, till all holes are drilled.
Cut through on the outside of the scribe mark and clean up , while spinning, on a linisher. looks like a fat, bicycle gear sprocket.
I also drilled the 2 magnet holes on the spindle.
cleaned up the "hole" edges with a fine round file.
This fits neatly in the hole, if it doesn't spin it on the linisher a touch more. It needs to be able to spin freely, on its axel. The thickness of the gasket paper is enough clearance, along with the axel, being fractionally thicker than the material thickness and paper together.
The first one I did, the dia of the holes was too small. I was using 3/8 tube, so needed to get a larger than 3/8 hole, as only 1/2 the dia is being used for the outside of the paddle wheel. 5/32 was enough for my needs.
When clearing it on assembly, I blew compressed air through it and it whistled like a top. Plenty of volume.
I got the idea from a wind measuring device. ( totally different in construction and not electronic, but the wind pressure was incremented by a "wind wheel" I had it hooked up to a fuel flow led display ( which, the whole thing has gone missing after my EJ20 conversion

Damn ! )