A little while ago I did this sketch for a correspondent who was interested in building a scale model (well, I hope it was a scale model) of the Nostalgia for Infinity. To my surprise (and delight) there are already quite a few sketches and models of lighthuggers out there on the web - some of them very close to my mental image.
You can view the image at full size by clicking on the version at my website. I couldn't find a way to link to a bigger one from here.
I semi-pinched the name "lighthugger" from Ian Watson and Michael Bishop, by the way. Their excellent first contact novel "Under Heaven's Bridge" has relativistic ships called lightskimmers. And look - it's available to download: