There are a few versions - Velo Orange do some and Freshtripe do their own...That's interesting, I have never seen these bosses for flat bars, that would be very useful.
Freshtripe mounts
Velo Orange
I think most bar end levers will mount straight onto the bosses but you will need a flat adapter for downtube levers.
If you search on AliEpress you can also find a similar type of mount on there too.
If you have indexed downtube or bar end levers you can retain indexing - I use the ones that can flip between friction and indexing so I can choose depending on my mood.