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Drill or preferably ream an 8mm hole into a block of something you have to hand. Also drill and tap the three M3 holes to match the pattern of the partSet block at the required angle in your mill vice and insert the turned and drilled part into it, fasten with screws. You can now plunge cut with a 15mm dia cutter or boring head.When done unscrew and move round to the next position, refix with screws which will take car eof teh indexing and cut the next and so on.Could also be done with a hex collet block if you can rig something up so it goes back into the vice at the same angle/position.
PS the drawing suggests it is more than just a 15mm dia at an angle due to the two lines that radiate out.
Is there a reason the air slots have to be angled and straight? It occurs to me that if the design was changed to straight slots cut with a ball nose end mill, you could machine them in the mill with the cutter parallel to the other holes. The flange under the prop would remain, to support it, and cut with a ball nose endmill would relieve the same or more weight and leave even more room for airflow than the straight slots. Just food for thought. Good luck.
I'm guessing 'out of stock' is indicative of your predicament that its an older gen motor & spare are getting hard to find?https://www.modelsports.com.au/product/axi-model-motors-radial-mount-set-for-axi-41xx-xx-v1-series/
Actually this fixture could be improved with the part shaft facing up & just using the SHCS bolts to both retain & locate into clocked position. Bolts aren't really proper for accurate indexing because of the threaded OD acting on the surface of the hole so you'll have to make a judgement call there. Is the circular relief notch just a continuation of the cooling vent hole in the motor can or does it need to accurately mate something & becomes important for concentric prop shaft running? For better accuracy you also use 2 close fitting dowel pins to index the 2 bolt holes & then some kind of strap clamp to secure the part. Its the old fixture time vs. part making time HaHa. Hey, if its a rare & desirable part, make a few more while you're at it more & flog them on Ebay or RC forums!