Good catch!
the tank should be stable foward and side to side, up and down also . there is not much to cause it to slide backwards though.
foam is fairly lite but rechecking balance after any changes is wise. while you have the tank out tie a string around it for easy removal next time.
1/2 " of foam is plenty but you may not have that much space so do what you have room for. don't pack it to tight or the foam can't do it's job.