I would recommend that you take some anti emetic pills instead of closing your mouth with duct tape. Anti emetics will make sure that you don't puke. Using duct tape can have some adverse effects, it won't really stop your digestive system from releasing all its crap so you will puke but if the puke doesn't come out of your mouth, you'll end up choking on your own vomit and that'll be painful. In a not-that-worse case scenario, the duct tape would just come off. For the Xanax, just check the LD50 and double that number and adjust it according to your mass, I think that should be enough.