the battery monster has struck, will get new ones when i go out tonight and take pics, but should not be to hard to imagine.
cut plastic strips for the volume of rectangle you want, super glue them in a few spots to lip of the tank so that the top of the strip is level with the top of the plastic, then fill in the space with silicone, cut off access when it cures. Now the gas bubbles into the box that is formed inside the tank. Some hang on filters will sit funny, others will not.
I am figuring eventually I am going to have to purge the gas from under the lip as it defuses Co2 into the tank and is replaced with oxygen and nitrogen from the tank water. I used a tube to suck the initial air out of the box so that the water could be displaced by Co2. I doubt the osmosis will happen at a rate great enough to keep up with the production from the yeast reaction which is where the powered reactor may be superior.