well something ive been doing lately to help my punching is,i get a small weight (im using a 5 pound barbell), or a small weight relative to you, nothing heavy like 20 pounds or anything, but something that you feel has mass to it (get what i mean?)
and basically what i do is hold it and shadow-box with the weights in my hands.
what that does, it helps build hand speed(you said you already had that, but extra training never hurts) and also, as your shadow-boxing with the weights in your hand, its working out all the muscles used in a punch because youre putting resistance on those muscles, so its like when youre working out any other mucle, so basically it works out the power of your punch. and as you get used to a weight, you can always increase the weight. hope this contributed to your self-standup workout.