I have found two viable solutions:  
<a href="https://www.youtube.com/watch?v=rXce7MSlEws">Bi-directional Pistonless Counter</a>  
<a href="https://www.youtube.com/watch?v=etVGAF2p7zk">Signal strength memory</a>  

The second one is faster, more compact and outputs directly as signal strength while the other has 15 different binary outputs.