http://centroeu.com/uboote/117k.jpg
http://centroeu.com/uboote/116k.jpg
http://centroeu.com/uboote/254k.jpg
http://www.uboataces.com/photos/2005992315062578.jpg
http://www.uboatarchive.net/U-604Binn16.jpg
http://www.ubootwaffe.net/ops/boat.cgi?boat=354 (bottom right image)
Also have a look at the first link in Pant's post, which gives a reasonable closeup example of a few (they are WWII pennants despite the rest of the comments).
The design was very much up to the creator, based on what material they had at hand, although including the tonnage amount was standard. A lot of photos I've seen also include (presumably) the ship's silhouette. Of course, they were all triangular.
According to a number of references I have ("U-Boat Ace" by Jordon Vause Wolfgang Luth comes to mind), they tended to mostly be on white fabric, with some having coloured borders (for example the right boat in the second last link above).