I’m telling you this because, in a few months, you might be me, and we will want to take advantage of some of the cunning plans that we laid in the summer.
We’re going to attempt to fold the crossheads up from etched material. I’ve put four of them on the etch because there’s loads of room and because it’s likely we will make at least one mistake. That’s them there, on either side of the connecting and main rods; they look like little men.
The trick with the crossheads is to fold them while they are still connected to the fret. Then the holes that accept the piston rods will be positioned across from each other. The plan is to pass the piston rod material for two crossheads between the ones on opposite sides of the etch; this should ensure the piston rod is perfectly straight relative to the sides of the crosshead.
Good luck, future me!
Pembroke:87 
Your series on building this model is fascinating Rene, especially as you move ever closer to metal work. I hope you can illustrate the folds step by step at some point. The mechanical considerations that result in a smooth mechanism are easy to summarize in simplified terms, but when an author get’s into the details – as you are, thank you – I sometimes lose sight of the rational for certain steps. In this case, what alignment (in what planes) matter is kind of eluding me. More please!
I’ll try, Rob, but I expect once the frets arrive, I’ll be disinclined to stop for photos! Too exciting!