Print two off.  After printing check the patency of the three holes across the width against your steel rods, which should be able to be inserted without too much force.  Gently, thread the screw holes at the ends (6BA/M2) for your chosen screws.  You need to be able to remove at least one W-iron from the bolster to insert/remove the wheel/axle assembly, but one W-iron can be glued in place if required.