this might be of interest to you.
there's a cheap behringer clone of this one if you want to practice your modding skills on something more disposable.
https://www.youtube.com/watch?v=a4DCPuGyUqAi find the 'feedback effect' a bit synthy. up to maybe a second long it's hard to tell it's not real feedback unless you are listening out for it, but the longer it sustains the more square wavey (with a hint of octave) and fake it sounds to me. you can get a better result with a booster on a stomp and getting up close and personal with your amp. but if you like that efffect then there are quite a few proposed mods on the freestomp page you can try.
http://www.freestompboxes.org/viewtopic.php?f=11&t=22119google has the schematic for the boss. take that and apply it to your boss or behringer.
i think some of it is digital so it wouldn't be easily buildable, especially for someone new to building. and if you can find a behringer for £20 why torture yourself?
but it's a fun thing.