Are you talking about sliding a piece of acrylic in the slot where the current cartridge goes?
What I did was pull that cartridge out. I drilled holes in the very bottom of the media chamber then clued a small piece of plastic over the slit on the right each. Then stuffed that entire chamber full of filter media. That way water has no choice but to be pulled through the filter media and out the bottom. I but a bunch of Bio balls in the second (big) chamber with the pump. If you decide that first chamber in half its going to reduce the size of your filter media. I drew you some pictures but can't upload them from work.
Yes. I like your idea, but I am a little confused about some things....I'll just wait until you upload the drawings later.
Also unless yours is different then mine, the media chamber doesn't go all the way to the bottom like your drawing.
You are right, I just realized that.