Sometimes, it can be a simple as changing the leg in which you wear the Ocean Guardian. If you wear it on the leg that you kick straight up outside of the water first, it gives time to basically drop near your head. By switching it to the other leg, this can sometimes be eliminated. A lot of it depends on each individual’s duck dive technique.
If the above doesn’t work, you can drill a small hole in the outer edge of your fin (towards the end and near the rail) and thread a cable tie loosely through the hole. Then the antenna can be threaded through the cable tie and this can keep it from kicking back. Carbon fiber fins can be drilled – the dive shops drill them all the time when they put new blades into pockets.
A float attached to the tail end may also work, but it's more used for Scuba divers rather than free-divers as scuba divers spend more time on the bottom. A concern with the small float is that you could potentially be introducing more drag when ascending….if you’re coming up from 100ft, you do not want any more drag.