I'm not sure a slide lock would do much good if the top doesn't come in all the way right now. I would suggest you might need to adjust it so it will come in all the way and then purchase a slide lock. To answer your question, I don't know how to adjust but someone will come along here pretty quick that does know how and will give you some help.
2012 GMC 2500HD D/A Crewcab
2012 Jayco Eagle SuperLite HT 26.5 RLS
2009 Crestliner Superhawk 1900
2013 Polaris Sportsman 550 EXP
2008 Polaris Sportsman 700 X2