Luis-I've only ever done one brisket and it was a 5 or so lb flat from Costco, similar to what you describe. I used the HH version exactly as Scottthegrillinfool describes in one of his posts in the Photo section. It did not turn out well (nowhere near tender enough).
I can't say for sure what portion of this was my fault-may have even been entirely my fault-but I do know that feedback I got in my HH Brisket Diagnosis Request in the bbq section was nearly unanimous: learn brisket with a packer. Obviously there are at least two people right here on this thread who can do flats just fine, but dang near everyone on that thread, that I recall, strongly suggested going with the packer. Way more forgiving, especially on a HH cook.
Just my 1/2 cent of advice/feedback