When I'm making a pep that is closed, I always make the last piece I put on a tabless piece. That way, I get a good amount of glue on the tabs, put the tabless part on and make sure everything lines up as the glue sets. This usually means that the tabs will not be completely sitting against that last piece, but the excess glue holds it in place just fine. If I have some time, I'll post a quick tut on that.