D Seat post rear light

ive just bought a giant defy which has their D fuse seat post. I’ve no idea if this is any more effective than a regular seat post but one downside i have encountered is mounting rear lights. I just tried to mount an exposure trace r light and whilst it does wrap around the seat post the mount assumes it will be flush against a spherical seat post which is not the case with the D style post. The light therefore rotates horizontally during the ride which is not good. 

Can anyone recommend a rear light (or mount) that will work with a non standard seat post? I’m thinking to use a saddle rail mount but this will mean losing my saddle bag

many thanks

Toby