Travelers Company brass ruler
The way I use my A5 Hobonichi Cousin is that I split the day pages in three. Just to be able to fill more stuff into each page. Plus that my todo items are more 1-3 words than full sentences.
Until now I just freehanded it, and it looked like shit.
So I finally got around to buying a ruler. Of course I bought a nice brass one. I wanted one from Ystudio but it was out of stock, so I got one from Travelers Company instead.
I like it a lot. But it is a tiny but too short. But the good thing is that it fits in the case I carry my pens in.