I revised my design thanks to snakes advice yesterday
, I don't have the drawing on hand right now, I just added another baffle from the refuge area. For the overflow issue, you can either have your tank drilled, which is the best option in the long run, but is rather difficult to do. The other option is a having a hob overflow, I heard some bad things about them, but I also heard some good, just do a search on them on the forum, you will find lots of info on it. With your 20L being glass, your best bet is making your baffles out of glass, and siliconing them in place, silicon doesn't adhere to acrylic, so your not doing anything more than wedging a piece in there between silicon, it will eventually come loose. My design is based on my protein skimmer foot print, and my tank size, so with your tank being smaller, I would suggest shortening up the refuge area, make the skimmer big enough to house your skimmer your planning on getting, and make the return area hold as much volume of water as possible. Like snake said, you should use an ATO(automatic top off), it makes life so much easier, you no longer have to worry about your tank water getting low, or your pump sucking area and pushing bubbles into your tank! If you have any questions just ask me, or anyone on this forum, they are all very knowledgeable!