Tell me why I can’t make a fixie hub by squirting glue in a normal freehub?

I gather its been done using araldite or plastic metal before

Glue? This is a joke right? All of the bodge welded S/S freewheels I’ve seen have eventually let go. Glue/Araldite/Plastic metal will in no way be strong enough to hold up a freewheel as fixed hub. Ever tried to remove a fixed sproket from a resonable rider’s bike? The force needed to remove will be a good indication of what it’ll need to hold.

You could always weld it up.

Or braze it like Sheldon.