Coffe filter in a pinch.
If you can find a silkscreen supply place they will have many nylon mesh choices for you.
Get a wide mouth plastic jar, cut the bottom out, cut out the center of the lid, stretch your 50um screen across the top & screw on the lid. Ta Da... plankton screen
Find some scraps of 6-8-10" pvc, check around highway construction. Use pvc glue to attach your screen. Again, Ta Da
Thank you, thank you... I'm here all week.