I managed to get the last required meassurements for a first prototype of my own extruder design.
Not printed yet and certainly not tested yet.
The magnetic encoder can be mounted above the drive pulley. The head of an M3 capped bolt inside a 3mm ball bearing (6mm outer diameter) is driven by the filament streaming by.
The encoder-board is held by 3 M3 bolts driven directly into the extruder-block and cutting their own thread.
I think this will hold securely but this remains to be tested.
The magnet is glued to the bottom of that bolt, sticking out of the extruder, and rotates below the board.
In the design it may look like this but in fact this M3 bolts does not get in the way of the M5 spring-loaded fastener bolt. (I hope.)
The mounting at the bottom is only for the Thing-o-Matic in this early prototype.
The later version will also allow other hot-ends and I'll see about an adapter-plate to mount 2 of these extruders (one mirrored) in the limited space of a Thing-o-Matic.
2 of the M3 bolts use trapped nuts, the other 2 are inverted and need to face downward instead of up. (not enough space below the pressure-arm.)
Update: I did a very rough prototype at 48mm/s for the first tests.