The pattern is easy to make based on a tried-and-tested t-shirt pattern and it's easy to sew as well. Easier that a proper cowl-neck even.
You start out, obviously, with the bodice pieces of your pattern. If your t-shirt block has bust darts, it can be easy in this case to rotate them to center front.
Draw in the new neckline. I made it about 2 cm wider at the neck (starting from a high, close fitting neckline) and made a long, narrow deep scoop to about 10 cm above the waistline. This goes through the bust point and should include any bust darts there may have been. Draw a new back neckline to match the front one at the shoulder seam. Only lower its point at the center back line as much as you need to make a nice curve (no more than about 0.5 to 1 cm).
Measure your new neckline and use the measurement to draft the drape piece. It's easiest to just draw a horizontal line first, in the middle of the paper, of the length you measured. Then, draw the center front and center back lines at 90 degrees from that line (only draw one side). Mine are 24 and 8 cm respectively. Draw a gentle curve between them as shown.
Now, measure along the curve and adjust the beginning and end of the piece so the curved line has the length required for the neckline of your top. Add seam allowance if you like, fold the paper along the horizontal line and cut the drape piece out through both layers. You should end up with a pattern piece which looks like the one in my drawing.
When cutting fabric, both the bodice pieces and the center front of the drape piece should be cut on the fold.
When sewing, start with the shoulder seams of the bodice pieces and the center back of the drape piece. Then, fold the drape piece double with the wrong sides facing each other and insert it into the neckline, treating it as one layer. Because the result drape neckline is smaller than the seam line, the stitching won't easily be exposed.
Of course, you can make lots of variations by changing the size and placement of that draped neckline. Or you could put the fullest part of the drape somewhere else than at center front. Or you could also insert a piece like a dickey into the neckline, keeping the top even more fitted and eliminating the need to wear something under it.
And although this is a design for jersey fabrics, there is no real reason why it couldn't be made in a woven. It would just require quite a bit more messing around with darts and you should cut the drape piece on the bias...
I hope you like this tutorial and everything is clear. Don't hesitate to ask questions, just make sure I can reach you to give an answer (if your blog or email address in your profile or comment, I'll use that. If it isn't, I can only answer here in the comments so make sure you check. Or email me instead, the address is at the top right of this page) And as usual, I'd love to see your results!