Ok just figured out something...
my original thought was that tying the screen would help in the cleaning process...just take the entire rod off along with the screen, scrub it and return...but I just figured I will need a way to keep screen in the air flow's path....maybe have the entire system to be removable? I mean secure screen to rod and airstone using zip ties or similar...just disconnect airstone from pump when need to wash and replace/reconnect when done...
I liked the epoxy idea proposed by Floyd to cover up all the sides of the stone thereby directing flow upwards only...would aquarium safe silicone be a good alternative? dunno about finding safe epoxy over here
so basically, to get back:
airstone sealed at 3 sides, leaving just the top side open to allow air out.
screen fastened to stone at the bottom and rod at the top.
rod rests on sumps edge...
since its all still on the theoretical side, im thinking if the first chamber will have enough of a water column to cover the screen up, since it will be the in-flow of water from the tank... maybe if i made the screen wider (40cm) with less height...SM would there be a difference impacting growth? I mean the intent is to have water covering the entire screen right? if water column is not too high, then i guess compensation would have to be done with the width to achieve desired surface area...