The D-cells each heat their own tube, as the 3S4's are directly heated and cannot share a filament supply in this circuit. When I worked up the layout, instead of using one screw in each of the four corners of the D-cell holders and two extra holes per D-cell holder to run the wires through the chassis, I went to two screws per D-cell holder and slipped the wire leads through the other two holes. This minimized the number of extra holes to be drilled, and if you wanted to put the batteries inside, these holes could easily be filled with extra screws to hold the D-cells in.
It is kind of funny to mess with someone not familiar with the Quickie by flipping one of the D-cells around while it's running...