I don't have a pattern to suggest... But... You might want to try what I have done in the past. It was actually easy and looked great.

I took a bathrobe that fit the person I was sewing for, and laid it flat on some paper. (Old wrapping paper works great, so does butcher and medical paper.)

I took a yardstick (Any long, straight stick will work.) and angeled it out from the armpits to form a triangle with the bottom of the triangle being the bottom of the bathrobe.

Trace onto the paper the neck, shoulder, around the arm seam, and the angled yardstick. Trace the bottom of the robe, or make it longer if you want.

Take the yardstick and move it to the inside of the bathrobe. Line the top of the yardstick up across from the bottom of the armpit of the robe. Angle the yardstick outward again, making another triangle.

Trace this onto the paper, following it up to the inside of the neck of the robe.

You now should have 1/2 of the robe traced. (With two lines around the neck, one for the back and one for the front.)

Round out the bottom of the "pattern" so that your triangle becomes a cone. (Think upside-down ice cream waffle cone and you'l see the kind of arc that you want on the bottom.)

Add 1/2"-1" all the way around for seam allowance. Your choice, just keep it consistent.

Cut out two pieces for the front. (Remember to flip the "pattern" over so you have opposite sides.) Remember the lower neckline is for the front...

Cut out one piece for the back, if you are able to fold the fabric first. If not, cut out two pieces, you'll just have to sew them together to form one piece... Remember to use the higher neckline for the back.

Sew the side pieces to the back pieces from the bottom of the armpit down. Remember NOT to sew the arm holes together.

Sew the tops of the shoulders, front to back, together.

Fold the edges over twice, using small folds, and sew them flat.

Use any belt you like...
Add your closures...
Viola!

(Hope this helps! Enjoy! And post the photos!!)