All my essentials like wallet, phone, disc lock etc for a short day out go in a Givi Tanklock tank bag, that way it's a super quick on and off for things like coffee and petrol fill ups.
Got to say, I was against top boxes as well since I didn't want to look like a pizza delivery boy. Got the Vee which basically has a walk in wardrobe on the back and wouldn't ever go back. It turned a toy in to a mode of transport and meant I used the bike way more instead of the car. I'll never walk around carrying my lid or wondering who's knocked it off my mirror ever again 😉