I own many many guitars & amps (don't even ask how many), 5 of them being cigarbox, having self made CBG to 2 Daddy Mojo's(a 4 string and 6 string resonator). I also owned a boutique guitar store for many years, I was the first dealer for Daddy Mojo and dealt exclusively in the "Ferrari's" of the guitar world (Suhr, Caparison, Framus, Grosh, D'Aquisto, Trussart, Bad Cat, Bogner, CAE, Etc)
A few things to simply your first build.
1. Make a 3 string, it's the traditional cigarbox and also easier to learn on. Once you get the hang of it, move up to a 4 string.
2. As far as the neck being round, it only needs to be round if you plan to play it held and not in your lap. Most that I own I play in my lap and they are square necks.
3. As far as scale length, 24.75" is short scale and used primarily for a spongier feel to make string bending more easy. 25.5" scale length results in higher string tension. Long scale also results in more volume due to the higher tension. Unless you are using a hard wood (maple, etc) go for short scale. Also keep in mind there are MANY tunings you can use, some of which put an excessive amount of tension on the neck. Make sure the wood you select for it is defect free.
4. Frets, I would suggest not fretting on first build. Without a few specialized tools for dressing and leveling the frets they will most likely be uneven and result in buzzing, etc. Your build tolerances also have to be more precise due to the use of frets, making minor errors turn into a major problem. When fretting, measurements are in thousands of an inch to give you an idea of how precise.
I went in way over my head with the first build I did many years ago and end up scrapping it. I tried a 4 string fretted, without any experience in building my own guitar. If you just go for a simple 3 string non fretted, it will be a much more enjoyable process for you first build. My 2nd build was a 3 string non fretted and a very enjoyable process.
