Yes a knockout stool flange, toilet, wc, would work if butted together with a gasket and have bolt holes. There's a few things that could be modified or even just bolted together like a flange fitting that could be gasketed and screened with appropriate size to facilitate the holding of the filter media. I know they make flanged pipe adapters but availability in the sizes needed would determine a lot of things. Not my best engineering but with a little work I think it would be quick and efficient there probably are many things like screens built into flange gaskets for other industries like brewing, oil production, etc. Working on a similar thing to catalyze gold and pgm recovery from ATS and flow at a reasonable rate.I think a flange with a blanking plate would be best for the filter plate, with lots of very small holes drilled in the blanking plate. A shower drain cover usually has quite large holes, no?
Perhaps "quick release" bolts could be used to secure the plate to the flange- the type used for bicycle saddle stems etc... Maybe also an idea to modify the flange and plate to have slots rather than holes for the bolts, so the bolts can be inserted sideways rather than pushed through, though this might compromise the strength of the flange a little.