Exit the vessel via opening an EVA hatch and walking out through it.
Place trigger colliders just at/outside the hatch opening and trigger EVA on contact.
May need a second larger collider to trigger a screen message warning that you're about to go on EVA.

I think it might be more reasonable to immediately activate "eva mode" when unbuckling, but modify the kerbal's layers/colliders/etc for IVA movement until you pass an external hatch.