I recommend the muji notebook it's similar . Slim and has a cardboard cover. They come in a variation of sizes as well as options for line or grid paper. MUJI Notebook A5 6mm Rule 30sheets - Pack of 5books [5colors Binding] https://www.amazon.com/dp/B00I6Y0MH8/ref=cm_sw_r_cp_apa_i_.ASwBb99206A4
The Pilot Metropolitan fine is very fine, the medium is a good nib to bring out the greatness of fountain pen inks.. Although, a fine nib may be better behaved on cheap paper.
To get fountain pens you kind of have to sacrifice paper, you won't be happy with mead paper.
These are the notebooks I use they're small so I try to replace them after each unit test
i think they were the Muji A5, A6, B5, or B6 7mm lined notebooks. they are pretty cheap in stores. online the prices go up because of shipping or resale.
here's what it usually looks like.
Muji makes thinner A5 notebooks that might work: https://www.amazon.com/MUJI-Notebook-6mm-Rule-30sheets/dp/B00I6Y0MH8/ref=sr_1_2?ie=UTF8&qid=1503364878&sr=8-2&keywords=muji+a5
and a thicker one that's slightly more expensive https://www.amazon.com/MUJI-Paper-Notebook-Rule-72sheets/dp/B00IQUFBJG/ref=sr_1_5?ie=UTF8&qid=1503364895&sr=8-5&keywords=muji+a5
These very thin, 30 pagers of good quality paper that you can use both sides with. I intend you to one per each unit.
These are very similar and equally as good I don't have a preference of the two but I really like the dot/lined setup.