I go to the distributor where they're around $3 for magnets and $5 for the 1" thick 1 screw slabs. I'll see what the largest they have is. Someone needs to start a sticky with a list of all products and the point thickness. I always end up with a litle too big. I'm buying one of all sizes next time to use as reference.
Dont use the hinged cases, like mentioned, the corners are rounded on the inside and put the pressure on the card corners.