The 'Art Book' for the film has a couple of good photos of the actual prop made for the film. It's even 3D printed (and you can tell..)
So I thought I'll whip one up because I can. Very much a work still in progress - as I type this the UP Box is whirring away making the first set of parts. Only got half of it out so far but everything fits together as planned.
No time yet to do the interior parts, but you don't see that stuff in the film, so it can wait.
It's big of course, so you may want/need to do it half scale, both to fit it in your machine and on your shelf when done.
as fine as possible for the rollers and roller bases
3D Fillies ABS
The "Shell" parts print well standing up on one of the flat edges
I was pleased to see this all stays together without glue, as long as you don't swing it around too vigorously.
Only five different parts. You'll need 6 each of the rollers and roller bases; 8 shell segments; 24 finger-hole surrounds; and 24 pegs.
Join two shell parts together with the little brick-shaped pegs; then another two (orientation of the finger-holes is random, not symmetrical). Then push the two pairs together either side of one of the roller base rings. Repeat this so that you have two half balls; then insert the remaining four roller base rings into one of the halves and join the two halves around them with eight more of the pegs. Finally snap the six rollers onto their ring bases; and push the finger hole bezels into all the holes in the shells..
And her she is conveniantly holding it right beside her head. Doesn't look quite 30cmm to me. Printing at 75% might be better? But then the fingerholes are tiny, even for someone as small as Alita.
Photo of the actual prop, from the book "Alita Battle Angel: The Art and Making of the Movie". Lots of stuff inside to do next time I have a long weekend..
According to the book, as in the manga, the motorball is 30cm in diameter. I've assumed that means the red bit is 30cm (but now that it's coming together, I'm starting to think perhaps it's 30cm from tip to tip of the silver roller-balls?) Next I'm guessing the six rollers in their holes are about 10cm and the red shell looks to be around an inch thick (metric inch). I think maybe I made the finger-holes a bit big on first attempt, so have added a pair of extra files - Motorball Shell 2 and Fingerhole Smaller - which I would recommend using instead of the earlier versions. I've also very slightly widened the V-groove between the segments. Small tweaks but looking just that little closer to the original, I think.
And of course it's designed to allow addition of the internal detail in the future that never comes.
Solidworks of first version