Venting a chimney stack at roof level

hi guys, has anyone had a previously capped chimney stack which required venting?
I don’t want to remove bricks as they don’t line up with the flues.
I was thinking of either using a core bit to install round vents to each individual flue, or drill out perps install a weep vent.
I know the weep vent won’t have a great effect so I’m leaning towards the round vent. Just wondering what size I’d need as I don’t particularly want 4 no. 100mm vents on the side of the chimney.
Any ideas please.?

Whats wrong with a standard clay air vent?
I don't know what you mean by flies

Thanks for your reply catlad.
I’ve edited post, flies was flues!
A standard clay air vent would mean removing bricks. And the bricks don’t line up with the chambers.

why not post pics of the flaunching and the stack?
also, why not read up on here about whats needed for redundant flues and stacks?