I'll second the enclosure being too small. It's like you living in your closet your entire life. Just take this in and think about it.
Everything I've ever read indicates 40gal. is minimum for an adult beardie. Do you really believe everyone but yourself could be wrong here? People who make a living raising beardies? People who have raised beardies for far longer than 5 years? People who have spent thousands of dollars on education alone regarding the husbandry of these types of animals? You truly believe what they say is wrong and you are right?
I can tell you this, if you do not listen to what the experienced folks here say, your beardie is going to have an unhappy life. Do you love your bearded dragon? Do you think it's fair to the animal who has no choice to live any other way other than the way you provide to be cramped up like that? Seriously, just shut yourself in your closet for a day and see how bad it bothers you. That will give you an idea of how much it bothers your dragon to be in a 20gal. home. Even worse, tell someone to force you to stay in there for the rest of your life.
Please don't take this post as an insult or an attack, take it as a plea on behalf of the animal you care for. Take it as a request from someone who has compassion for the way your beardie feels day to day. Take it as a sincere request to give your beardie a happy life since it's life is entirely in your hands.
I truly hope you reconsider giving your beardie a more spacious place to live and wish you and your beardie the best for the future. Just remember, how BOTH of those futures will be are completely up to you.
-----
Rob Talkington