Thanks for the suggestions, and the offers of M8 threaded rod and the loan of bearings. I think I've got all of the parts I need now (although the ciclop board is taking a slow boat from the land of Banggood).
I'll start with a 3D printed bearing frame (possibly with some laser / hand cut sheet material to give the balls a smoother ride top and bottom). I may well be interested in borrowing a bearing though to see if there is an impact on the quality of the scanning.
I think the laser lines I have a little wider too (12mm diameter) than the usual ones so there's going to be a bit or tinkering to do with the models there.
Other than that it's just a question of working out which safe place I put that stepper motor I was planning to use in, and cracking on with the printing and building.