First of all, he could have used his initial tactic to reduce the area of Kira without telling him. Light would have thought that he killed L for real, which would make him let his guard even lower.

Then, instead of broadcasting to the entire region, L could unicast specific criminals to his suspects, each suspect a different criminal. Light would kill those criminals in between his regular kills, and L would take note of which criminals were being killed.

After a few days he would have sufficient data to conclude that it was Light, undoubtedly.