/cgl/ - Cosplay & EGL

Let me preface this by saying I have never made a bra before and I'm also a guy.

I'm trying to make pic related for my girlfriend. A bikini top that's secured at the front with a square knot.
Image is from this tutorial video I found to make this top;
https://youtube.com/watch?v=fgj9yOO-VTE
Seemed pretty straightforward, only thing is the girl in the video has relatively average bust and the top sits nicely on her.
My girlfriend has J cups. Even after adjusting for her cup size, it simply doesn't sit nicely on her. It's like trying to wrap a sheet of fabric around a ball; wrinkles and folds every which way and it just looks ridiculous. There is very little coverage at the square knot, naturally since the breast portions of the top are being pinched off at the middle.

I really am clueless as to what I'm doing here, I've hacked at this for over a week straight and have nothing to show for it. Do I have to create bra cups? Do I need to integrate underwire? Am I even able to secure the front with just a square knot?
I'm at the point that I'm willing to spend money on a premade pattern that has been proven to work on large cup sizes.