Tip on cigar box ukes
O.K. I'm not bragging, so don't take this the wrong way. I am a self taught luthier and admittedly I have a lot of learning to do.
That being said, I have built approximately 120 cigar box ukulele's over the past four years, and I find that the old boxes from the 1950's and older work the best. They are typically made of redwood (a natural tone wood) and are not plywood. Also the box bottom and box lid thickness is right for a ukulele, so there is normally not any planing required to get a sweet resonance. I recommend internal bracing along the sides, front and back however because the wood is susceptible to cracking. I use cedar for internal box support because it is light and also a natural insect repellent.
If you are thinking about building one, I recommend the following boxes. Windsor Palmas, Don Rey, 1886, J.E.y.Cia Corinas, or something like these.
Newer non-ply boxes that also work well: Cohiba, Acid, EPC, Torano, CAO, Hemmingway, and the small Onyx boxes. The newer boxes are usually thicker, therefore require some planing or coarse sanding to thin the soundboard out.
Some decent plywood boxes: Padron, Punch (wooden boxes not the cardboards), and Ashton.
Be wary of boxes like Macanudo,and Romeo Julieta's. They are not plywood, but the wood they are made of is soft and spongy and doesn't sound too great unless you add a pickup.