Magnets can work, but if they’re too strong they can stop the air assist fan.
If you’re using a full sheet of PG, taping it to the front and side edges of the tray will help.
Most of us use these: Honeycomb bed holdown pins But of course you need to be able to cut through materials in order to make them, so it might be kind of a catch-22 if the cleaning and focusing steps don’t help. 
Keep us posted!