Since my boyfriend and I are roughly the same size I only have his size small shirts to work with on my size large frame. Which means I only have enough material to make a top and not a dress like the model in the video shows. I came up with a kind of cowl neck halter top. The buttons are in the back, which I think is really cute because you can leave them unbuttoned and show a little more back, if you are so daring. The back of the shirt has become the front and the folded collar is your modesty neckline and then you tie the arms around the back of your neck. So it will leave loose material for your boobs to fit in and create a cowl neckline. I absolutely love it.
Of course you could always just go and get a bigger shirt but what's the point if its not your own boyfriends? Here is the video that inspired me, I hope it does the same for you.