Whilst the chop was drying I made a start on the scissor mechanism proper. A length of 20mm square bar cut onto two 500mm lengths, it turns out that was too long, so I had to cut them down a bit more, they're 490mm long now.
Drilled an M6 clearance hole in the end of ech of them, you can just see it on the other end in this picture. Here I'm filing the end into a semicircle.
I decided, looking at the slot and the physics of the thing that it doesn't need a full semi-circle at the end, it just needs to corner knocking off. The opposite side of the end, if that makes any sense, doesn't run against anything. Time will tell if this is enough rounding playing with it suggests that it is.
You then need a hole in the middle. This must be the same orientation as the other, if you drill it crosswise then youn can't mount it! No I didn't do that. It's another M6 clearance hole.
Bolt it up so it spins freely but doesn't wobble. I used a nyloc nut on it, though there's hardly any rotational moment on it so a normal nut would probably suffice. It's symmetrical in length, which turns out to be wrong as you'll find out later.
Hang the scissor mechanism to try out out and sit back feeling pleased with yourself. It may need a little notch cutting out for the middle bolt heads when it's closed.