I have tried so many masks for my 7 year old for sensory issues. I thought the center seam and nose wire were necessary to keep them on my kid.
But actually the best commercial masks I've found are these "reusable face bandanas" on Amazon:
https://www.amazon.com/gp/product/B088KQ4TY9/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Second-best are the VistaPrint masks, which do have a nose wire.
If you really want a center seam and you have a sewing machine, I used this pattern:
https://sewmuchmooreshop.com/collections/patterns/products/face-mask-pattern
It's free and the woman does a great YouTube how-to that goes along with it. I purchased adjustable ear loops from Amazon to use instead of elastic. In my case, after some trial and error I extended the "medium" pattern by one inch along the bottom to better fit my kid.