It's about time someone did a 90% boat tail pickup!
Get those corners straightened out and you'll be in business- I think there are a few pockets of detached flow slowing you down- literally! Maybe make some transverse curves over the edges to get a smooth surface. Tuft that, refine, then 'glass it!
You're getting pretty close to this little number:
Aero-Dynamic by
jeff868_2000, on Flickr