On my boat, I fly the American flag from the NAV light post on my RADAR/FLIR mast. (you can see the post between the antennas in my this photo in my sig).
Is that a "stock" photo of you boat? If so, a photo of your actual boat may help.
I am assuming you don't have outriggers, so I think you are going to have to add some sort of mount/fitting to your hardtop. Perhaps you could use one/two VFH antenna bases with a "dummy/cheapo" antenna or PVC pipe with a cap. Maybe even find a used antenna on CL, Facebook MP, etc.
Hack: you could slide a piece of PVC over your VFH antenna(s) and mount the flag(s) to it, so the stress is on the stainless part of the antenna rather than the fiberglass. I have ZERO idea how/if this would impact the performance of the antenna.