If you have plastic frames they will always burr between the boxes no matter what the space. The top bars are too thin to discourage this. "it is known"
“…that very practical Canadian bee-keeper, J.B. Hall, showed me his thick top-bars, and told me that they prevented the building up of so much burr-comb between the top-bars and the sections…and I am very glad that at the present day it can be dispensed with by having top-bars 1-1/8 inch wide and 7/8 inch thick, with a space of 1/4 inch between top-bar and section. Not that there is an entire absence of burr-combs, but near enough to it so that one can get along much more comfortably than with the slat honey-board. At any rate there is no longer the killing of bees that there was every day the dauby honey-board was replaced.”--C.C. Miller, Fifty Years Among the Bees.
“Q. Do you believe that a half-inch thick brood-frame top-bar will tend to prevent the bees building burr-comb on such frames, as well as the three-quarter inch top-bar? Which kind do you use?
A. I do not believe that the one-half inch will prevent burr-combs quite as well as the three-quarter. Mine are seven-eighths.”--C.C. Miller, A Thousand Answers to Beekeeping Questions