But I did design it with this ebay listing in mind:
linear bearing slide SBR16 (6 supported rails+12blocks) | eBay
As C1 has said I will probably add the triangular side pieces if there is an excessive amount of flex.
As for the lead screws I'm going to be using 12mm, 2 start trapezoidal screws with dumpster AB nuts.
Will 2 start screws make a huge difference over the 1 start for this size of machine?