I use earth magnets on my bios.
I use 20mmx3mm any thinner and they wont be strong enough.
I use a flexible superglue to glue 2 magnets into the top of the crown and two in the jaw of the latex mask.
Then, I put the other magnets onto the outside of the mask (using ONLY the magnetic power to do this, then using a marker, number them so you know which one is which, just incase the polarity varies due to the way in which you glued the ones to the inside of the mask.
The way that I used to do the next step works, but not as well as the way that I do it now, You take a small square of camping roll mat (slightly larger than the magnets) and glue this to the side of the magnet with the marker on it (do it one at a time then put back in place so you dont mix them up) then put the two magnets on the crown first with the mat attatched and put some strong glue on the mat and put the bio in place and apply pressure until the glue has set, then repeat the same process for the magnets in the jaw.
What I do now is the same as but instead of using camping matt, I put a small ball of epoxy putty on each magnet, push the bio on, this squashes the ball of putty to the EXACT size of the void between bio and mask, I then take the bio off and allow putty to dry, then, I use 2 part epoxy adhesive and mix some up and let it thicken and put a blob onto the dried epoxy putty, then put the bio back in place, the sticky adhesive will mark EXACTLY where on the bio the magnets need to be, so I then put some more epoxy adhesive onto the bio, I then wait again for the adhesive to thicken some more so there will be no chance of it dripping onto my latex mask (you could always cover the latex mask with cling film prior to starting to avoid **** ups.
Then I just press the bio into place and leave it to let the adhesive cure fully, then I remove the bio and tidy the magnets up with some more epoxy putty.
I hope this helps, when I get a chance, i'll take some pics so you can see what the hell i'm talking about lol.