If the font must be Arial, then the OP simply won’t be able to insert a slashed zero! There is no point in suggesting to type 2205,Alt-x or Alt+8709 to get a slashed zero in Arial. Arial does not have that character! This is a typical catch-22!
But Arial Unicode MS is an extended clone of Arial, and it does have the required character in its “Mathematical Operator” subset. Would anybody notice that the slashed zero belongs to Arial Unicode MS rather than to Arial?