For those new to cb fiddle building, permit me to share some hard-earned tips to get you started right.
1. Getting Started: If you want to build a full-size (4/4) violin, it is critical that you get the correct measurements for the string length from the nut to the bridge, and the height of the bridge must be the same as a real violin. Bear in mind that a violin has an arch top, which raises the bridge height. Your cigar box will have a flat top, so to compensate, you must angle your neck to match the correct angle of a violin. If you do not get this right, your strings will not be the correct height on the neck or at the bridge.
2. Choosing A Box: You want to find a box that is long and narrow; the body of a 4/4 violin is about 14" long and about 5 1/2" wide at the bouts. It is critical that it be as narrow as possible, close to a real violin, or your bow will hit the sides of the box when playing on the G or the E strings. Also, a real violin is about 1 & 1/2 " deep, and if you want to attach a chin rest, you will have to cut down the height of the box in order to fit it on.
3. Creating A Template: I know this next step is time-consuming and counter-intuitive for those who just like to jump in and build, but neglect this step at your own peril! Once you have chosen your box and adapted it for the correct height, (and this is how I do it: I take the box apart at the hinges, and cut out from the middle section all around the sides, so that when I re-section it, it will be 1 & 1/2" tall.) Next, I stand the box on its side on a large sheet of drawing paper, and draw the outline of the box. Then I locate where the bridge will fall on top of the box, which is determined by the correct tail piece spacing, copied from a violin. I draw the bridge to its full size dimensions. I then lay out the neck, paying careful attention to the correct height of the strings to the neck at the nut and the bridge, and also by getting the correct string length from the nut to the bridge. I transfer all measurements from a 4/4 violin neck so that the neck and fingerboard thicknesses will be the same for the custom neck I will build. NOTE: YOU MUST CREATE YOUR OWN CUSTOM BUILT NECK, as it will be longer than a real violin neck, to make up the difference in the length of your box to that of a 4/4 violin! Once I have drawn out my cb violin to its full size, I can then transfer all measurements for my build, and confirm that the neck is properly angled to be playable.
4. Building The Neck: Building the neck is without a doubt the hardest and most time-consuming part, but if you get it right, you will be rewarded with a beautiful violin that will actually play like a violin! I go the whole route, hand carving and tooling the scroll volute by copying from a real violin. It is critical that you make templates for the peg holes from a real violin, and transfer them over. NOTE: CAREFULLY DRILL THE HOLES SMALLER THAN THE PEGS, and use a round file to carefully enlarge each hole to fit the peg. Bear in mind that the peg hole will be slightly larger where the peg goes in than where it comes out, so take your time with each hole. Get it wrong, and you will have to plug and re-drill the holes, which is time-consuming and not as pretty! I create my neck out of 2" x 2" poplar stock, and splicing together two pieces to get the neck angle, but it would be better to use a 2" x 4" piece of wood to cut the neck out of in one piece. This would also make the neck stronger, but either way, the joint will be re-inforced once the neck is glued into the box. At the part of the neck inside the box, I cut away some of the bottom portion, leaving only a bit to touch the bottom of the box at the front and back of the inside of the box. This lets the box sound better and have more volume.
5: Finishing Up: Once you have the neck correctly built, the rest is easy! You will follow through much the same as building a cb guitar. I do use a piece of 14" dowel rod to build a sound post, set just to the back and right under the bridge. Soundholes can be made up of the same kind of things you'd use on a cb guitar, or you can make a "f" hole template from a 4/4 violin, and cut them out with a sharp knife. If you find that your bow will hit the sides of the box when playing on the outside strings, you may have to cut out some of the top of the box where the bouts would fall. Otherwise, if you have chosen your box carefully, it will be narrow enough to play without too much difficulty! You can create your own fingerboard and tailpiece and pegs, but I prefer to order mine for a more professional look, and you can find these sets online. EBay is a good place to get an entire set for less than $20.
Hope this helps, good luck with your build!