I hate Fungus Nats. I find Demetrius Earth (DE) effective but you have to re apply when it gets wet and applying can be messy at best.
I just came up with this a few weeks ago. Next NatMats will fit better as I was using scraps of left over material. I placed, cut to size, filter material in a bag with DE in it and shook it up until filter was saturated with DE. Then I just placed NatMat on top of my coco. I can just lift NatMat to water and DE stays dry. Working great so far as it provides more surface area to trap the nats then just sprinkling DE on top of coco or soil.
Will be making NatMats to sell as trial to see if n e 1 interested.