I love my Mugen, and it took me a bit to figure out consistency with the filters. But it's pretty easy to fold an Abaca filter consistently, to fit well. You can see the sort of angle you need in the photo from the article. I just make that fold, and then smooth out the crease on the opposite side. I wouldn't go through too much trouble for a negotiator for this brewer.

Btw, the Mugen's strength in my eyes isn't as a single pour brewer. Rather, it's being able to grind fine, and with precise pours, minimize agitation to prevent channeling. I brew cool, at about 192F, with a bloomless technique. Once I get the coffee to sink with the first 3 pours (50g each on a 285 to 17g recipe), I'll do one last large 50g pour around the edge on the paper. Then for the last 85g I'll rinse the edges down as it drains, with a couple quick and gentle center pours to break any channels.

Zero bypass brewers can be easy for beginners, but don't be fooled into thinking that they're boring. You can take them very far with some more advanced techniques.

The one thing is, I wish the Mugen had a larger hole for faster draining. I'm hoping the Orea v4 is what I'm dreaming of.