The DIY thing is tricky. Certainly the basic DIY thing is not going to work with both pickups and surprised that fernandes would suggest 2cm, that sounds way over optimistic and extreme skepticalism is advised on that. It could be,k technically (here are patents) possible and have even 'heard' one in a clip that was the middle driver...but be careful what you wish for.
Reality is that the sound of the sustainer when on is not the bridge pup with sustain but it's own sound as it is driving the string at the neck position.
The interferances are no small thing, the only thing that really protects the interaction of magnets is the distance. I've played enough with mine to get a feel for what would be achieved and it is nothing like as great as people imagine and if it were easily possible, people would be selling them already, no?
That said, the DIY model can be made pretty compact...but you need to be good at this kind of thing and wiring tends to be uber tricky....
My tele here has it's driver surface mounted and is thinner than the already thin tele SCn neck pickup it sits next to. The switching is such that regardless of the selector, when activated the bridge pup is selected and the power turned on and all coils both hot and ground are disconnected as the sustainer driver will generate signals in any nearby coil especially, even the ground plane if not careful.
If you can afford a commercial unit, I'd be doing that, same with making pickups, it seems 'simple' but it's not worth it for the hit and miss results and cost a lot more to tool up to do it well. There is an art to these things and the makers know what they are doing, there is an art to such things.
I'd also caution throwing everything into some 'uber guitar' project, they rarely if ever live up to the dreams and can easily become a nightmare.
I just happened across this so shy away from such threads, but if you'd like some more explanation or thoughts or want to float a few ideas I can be contacted by the email or PM on this forum.
I don't make these things for others, but can give some explanations as to my ideas. The sustainer project from my side developed further still, being more compact (infact invisible) and low impact on the instrument, but that like many things are not DIY projects and has for the time being been abandoned as it's simply not a cost effective commercial proposition at this point in time. There are a lot more intersting things than sustainers too, but just making a really good guitar is enough, remember a lot more can be achieved in spending the hours on developing the playing side of things than anything that these technologies can offer and always there is going to have to be compromise. Anyway, the offer is there...pete