Carlos, while you're on this little adventure, pick up some pinstriping tape as DKanger mentioned. Comes in several widths and once you stretch it across a given space, you have an instant perfectly straight line. Of course it's not as nice as having a nifty brass tool that will look nice on the bench!
Several folks have mentioned altering dividers and this works well for some applications. If you take a pair of inexpensive Starrett type dividers and cut one leg very short, resharpen and polish the other, you can do pretty well with it.
Have fun!
Layne
shown are 2 fotos. 1 shows a starret, and 2 i made. the starret is unique that the blade or scribe is vertically adjustable. it could easily utilize a softer marking blade.
the other shows a brass clamp i got on ebay for 9 bucks. i am going to modify this into a marking gage. the bar is 6mm square, about 300mm long. i intend to modify this to somewhat resemble one sam showed. to do this, a jewelers saw, maybe a drill press, and soldering would be the only requirements