I eventually did dig my way out of the Bad Night of Modelling. The computer had decided to fit the drawing to the page, rather than printing it 1:1 as I had thought. Once corrected, it was a simple matter to start over again. This time, I made two smoking stacks of metal, and drilled the connecting rods first, holding them in position with that handy piece of 1/16″ drill rod I made last weekend while I drilled the second hole in the frames.
The design for the frames includes three printed spacer blocks, which cunningly interlock with the keeper plate to hold the wheels in place. The original three screws that hold Percy together hold the frame together too.
Amazingly, it almost worked perfectly on the first try. I had to re-print the front spacer as I had not adequately allowed for the tolerance of the printer, or for the angle the metal frames make as they are inserted. However, the other spacers worked perfectly, despite my amateur duffing with the drill press.