make a tent. like a pillow fort with blankets for walls and a roof. i used to do it alllll the time with no doob tube at all. after smoking i'd just sit in there for another 5 minutes or so, and then get out really fast so no smoke escapes. it never smelled when i did that. so if you want to be really safe, combine that with the doob tube and open windows and i can guarentee you you'll be good.
but just to warn you... 99% of the weed smoking commmunity living in their parents house will get cought. i never beleived it myself until i got cought. i was getting lazy and not being so carefull, but it was inevitable anyways. its better to get it over with. now i just light incense and my parents know that im upstairs "chillin out" lol