Morgadeth; You could route your return to run across and down the screen at a shallow slope. You can even put edges on the screen so it's like a sluice box and the sides contain the flow.
My understanding was that this is a highly aerobic process and so it's better to have the mat actually out of the water and the water running across it. Sort of maximized by that vertical unit pictured.
In fact the ultimate is to have the water cascade across, then stop for a second, then cascade, then stop, etc., etc.. In Dynamic Aquaria, an excellent book that really opened my eyes and switched me to an aquaculture mentality from instead trying to run a 'sterile battle ground', he has the mats lowered into the water, and then pulled, then lowered, in a mechanical method if I remember correctly. It would be much easier to somehow have the water just shift back and forth across, say, two screens alternately.
Also his method for 'seeding' was to tie the matrix in the ocean for a few weeks. Then remove the multi colonized unit install it in the system and let it select for the best species. Novel!
I'm thinking that the cyclical air exposure and the resulting agitation both work to dramatically reduce the hair algaes as I don't remember that he had any issues with it. He seemed to only harvest mat occasionally, and then often, used the harvest as fish food that went back into the tanks.
SantaMonica; Thanks for resurrecting this. I saw your original posts and thought 'spammer up to something'.
Thanks again SantaMonica for getting me interested in this again!! Great 'study' series up there. Lotta work to post. Much appreciated.