google for it, there is a lot of samples there,
and no one mentioned it, but, from 2014 when you got your WP approved until now, when you got refused, a lot of rules in the TFW programs are changed, so in your cover letter you need to adress the concerns of the officer that refused your application, and setisfy him that you are a "star" in your field. a would advise you to attach a letter from your employer/company that praises you and your job, maybe it will help.
best of luck.