The pics show 2 different shovel guitars, one has 2 tuners left, the other has 2 tuners right, knobs and pickup look to be different colors, the nut on one appears to be L shaped, the other an inverted T shape.
its a slide-fretless so put the nut and bridge where they work for you, as long as the scale length is roughly between 22 and 27 (give or take) you're good for git and not going soprano or baritone (unless that's what you want).
Set the action height to what feels right for you...take some kite string, butcher's twine, old strings, cardboard, scrap wood cutoffs, duck tape, painter's tape, masking tape, whatever, and mock it up and put your slide on your finger and see how the left hand feels (or right hand if you are making it for left hand playing)
Now that Im back on the internets, heres the video of the production model. shortly after this starts, he discusses where the frets are on this one.
Default, i don't know what happenened to my reply from about 12 hrs ago,but a few ideas cropped up after seeing that vid , you can use that joint as a fret mark easily, you just need to move the bridge back until that join is in a suitable position, and make your bridge higher, the shape of the shovel where it transitions to the handle socket will dictate your action height more than anything else, the shovel Justin is using has a huge hollow, that without being bent, is going to keep the action high, if it was me, i'd make the nut as low as i possibly can, also the bridge, that large gap between the neck and the strings will, unless you have loooong fingers like Justin, will take a bit of getting used to, so reccomend keeping it as low as you can
im thinking of cutting the angle iron in half (or finding half inch) so it wont be so damn high. I dont need mine to be identical, and if i can make a few common sense changes like lowering the action I will. I have that same shovel sitting next to me right now, staring at me with the sharpie marks on it for my approximations. my plan is to somehow mark the metal with fret marks too.
Since I can post links now. heres the original video of the one that was hand made for him that is the basis for the production. https://www.youtube.com/watch?v=V9-ltPsbw9g
i think the first thing you need to do is go shovel shopping...
find one with a nice flat length from handle to halfway down the blade... ultimately this will determine the range of scale length you could make.
I went looking and I don't see in any of the pics/videos what you mean, could you point it out please? which vid/pic?
I'd say the long distance is because, like you he wanted the join to coincide with a fret, but also wanted the bridge to fall down the slope a little to lower the action, and if ke made it the 12th fret, the bridge would be much higher, but with less space at the nut