I just for back from the gym, and I had decided to use the Chest Press Machine as a triceps only exercise.
The handles are angled down, so I inverted my hands---knuckles facing one another, so that it would strictly hit the tris. And then I repped away. It felt good, and hits the lateral heads, especially the outside head.
*I thought about this a few days ago since the gym has no seated dip machine, which I do like to use