As Wanhao has shortened the screws on the printer since the initial release, you will need longer screws.
UPDATED Sept 04, 2016
Added a channel for the metal frame to sit in to reduce movement even more.
A remix of Azza's "Z braces for Wanhao Duplicator i3, Cocoon Create, Maker Select, and Malyan M150 i3 3D printers."
Just print the top_right, top_left and 6 of the balls from Azza's braces along with the 4 STL's from this remix to fit the i3 PLUS. Reuse the fasteners from the printer.
1mm to 1.2mm walls or 3 shells, and 50% infill @ 0.2mm layer height
M8 or 5/16" threaded rods ~420mm