How to create an item that cant be removed from offhand?

Hi. Im trying to create a two-hands weapon. when I hold it in my main-hand, an invisible  item (not air block) with no shape, no physical form no nothing but does exist will be set in my off-hand , it will replace anything in my off-hand. the thing is, if someone click off-hand, the invisible item will be taken out of the slot, which is not exactly what i want. I want it can not be taken out just like the icon in slimefun book, you can see it but you cant remove or replace it from the slot.

thank you so much!