Hey Justin,
The Setup and fish look great!
I also use the Matala high density (grey) mat in my sump and I've found that after about 2 years it starts to soften up, after about 3 years it starts to get really soft to the point that it stops holding it's form and after 4 years it pretty much starts to disintegrate with small pieces coming off of it (almost to the point of being as soft as pillow matting). The frame is a great idea, but when the matala starts to break down, I don't think it will be rigid enough for the zip ties to hold it in place. I know this issue probably won't come up for some time, but it might be worth having a spare sheet of the grey mat cut to size and ready to swap out just in case you start to see K1 getting through.