Since I'm so damn tired of getting no sleep before midnight these days (and getting up 6 hours later), I investigated the interwebz and found what the topic says.
Basically it's just a fan, some liters of water which you can improve with ice and salt and some meters of copper and plastic. My idea is to use a pump from my old tank to transfer the hot water into a second can which can cool down over night and be used again on the following day. Setting up ~50 liters of water every day isn't really economic in any way and I think that's the idea of building your own ac ... Talking about that: Making sure that both cans are closed is important in my opinion, since a warm (or hot, in my case) room will likely evaporate all water and I have to refill it every day.
As soon as I find the time to get the missing materials I'll provide pictures of my construction, so long please feel pleased by a picture of my tank :3