The slotcartalk is strong in this one.
The trick is (may not be a trick at all) is to center the metal washer and use a slow setting cement. Center it like a potter molding clay. A light touch as it spins centers it perfectly.
Then let the cement between washer and fan set. The magnets are easily positioned by eye, confirmed centered as it spins and can be set with a little cement if you want.
Boy, you're really shooting for big mouthfeel with the malt dextrin. Not something I have any experience with. 1.1 Wow!
Sounds like 12 hours of beer embellished fun if you ask me.