Quoted from AUKraut:

I took some Goop adhesive and mounted some small ultra-tin neodymium magnetics off of ebay to the back of the shields. Let the Goop dry overnight and then let the magnets hold them to the speaker grill. That way if you want to change the look or clean/work on the machine the shieds are not permanently mounted.
Here is the link to the magnets I used, you'll use 10-15 per shield, size 5mm*0.9mm:
ebay.com link » 5 100pcs N52 Strong Countersunk Rare Earth Neodymium Fasteners Magnets Gifts