This is a self watering reservoir for grow bags (appr. 15 cm/ 6 inch). I tested it with wicks up to 6 mm in diameter but for the small grow bags you don't need as much.

It has 3 parts: Reservoir, lid, spike.
You add the wick to the spike, cut a slit into to bottom of the grow back with an knife and add the spike (no problem if there is already soil or a plant in the bag). Any wick works, I used Orimerc wicks + the 3.8l / 1 Gallon grow bags from Root Pouch. 
It's easy to disassemble, clean or replace the wick.

Feel free to use the provided f3d to make remixes for other sizes.