I got this Idea awhile back and started to mess around with it today. I wanted to make a prototype to see if the idea would work and what do you know the first time was a charm and it seems to work perfect.
Materials Needed:
1. Bic Pen
2. Styrofoam Scrap
3. Aquarium Air Tubing
4. plastic covered paperclip
5. Small Suction Cup
6. Super Glue
7. Scissors
8. Plastic container
Remove blue (or other color) tip of the pen. With this removed hold on to the blue section only and push the ink tube end against a table to slide the inktube and brown part of the tip out of the blue part of the tip. Now replace the blue section in the barrel of the pen. Then cut off the end of the barrel that has the press fit end cap.
You should now have a tube that is restricted at the end with the blue tip but still alows air or water to pass through. It should look like this but without the paper clip and suction cup.
Now take the ink tube and tip and add a small drop of superglue to the rollerball tip so that no ink can come out anymore. Let dry. Then slide the ink tube inside the barrel tip first. Notice that when the tip gets to the blue section it seals it off.
Now take a piece of styrofoam and cut it to the desired size and shape for a float that will fit in the compartment you are adding the evap drip. Im adding it to a 14 gallon biocube so mine is cut to fit down inside return pump compartment.
Use the old pen cap clip to bore a hole in the center of the foam float so that you can insert the bottom of the ink tube into the hole.
Use a suction cup and plastic coated paper clip to create a mounting bracket for the pen barrel. And then attach one end of the air tubing to the blue section of the pen barrel. Should look somthing like this.
So now this is what you should have in front of you.
Now just assemble the pieces like this.
Connect the other end of the air tube to the bottom of a plastic container that you can use as a resevoir of filtered water.
Now it just needs to be installed and tuned. It is nothing more than a simple valve operated by the water level in your tank. As the water evaporates the water level drops and the pressure on the seal decreases until water passes though it and into the tank. This increases the pressure as the water raises until it reseals. The assembly should be mounted at a level where it will seal at the desired water level and then open when the water level lowers even slightly. This can be fine tuned by adding and removing water at different mounting levels.
Now im sure some of you are saying
"That cant work!"
But ive tested it in the sink and it works like a charm. Im going to be instaling it tonight and fine tuning it over the next few days. It may not be pretty but its going to be hidden anyway and save me alot of hassle on vacations.
Best of both worlds and you can adjust it 
