The external chamber actually is sitting on a little shelf attached to the wall behind the tank... I couldn't figure out how to get it to hang on the back with the parts I had... if you're into DIY'ing you could probably make some sort of hanger for it.
If you look at the last picture I posted, instead of elbows on the outside of the 'u', I used T-fittings. In this I used a press-in cap which I had drilled and attached a 1/4" quick-release tubing connector (you can find them in the hose aisle at Home Depot). I then ran some green drip line (clear airline built up too much algae) to the venturi input on my MJ1200 (it's part of the director nozzle that comes with it). This has the added benefit of drawing out any air bubbles that may get stuck in the tube before they have a chance to break the siphon.
The way the back part works is that when the water gets to the bottom of the elbow (like in an outage) there's still water in the bottom of the chamber, and in the inner chamber, so that the water stays in the u-tube (just like if you hold a glass upside down in a sink full of water and when you pull it up the water stays in it).
That way, once the water starts spilling into the internal overflow, the weight pushes the water from the inside chamber, through the u-tube, to the outer chamber. The elbow on the outlet tube really isn't needed... it's kind of my 'poor mans' durso... it cuts down on the gurgling to some degree.
One thing to keep in mind is that during shutoff, the water level in the inner and outer chambers will seek to level itself, so you have to make sure there's enough pipe on both sides of the 'u' that they stay submerged. Also make sure that the outlet pipe from the outer chamber sticks up so that some water is left in it (like how mine is with the elbow).
I hope I didn't lose you there.
I like backyard engineering because I'm a cheapskate...
If it seems too complicated you may want to just get a prefab one that you don't have to think about too much... you can still retrofit those with a hose barb or something so that the powerheads will suck start the u-tube's also, and you'll probably save yourself a lot of headaches.
I just thought I'd mention it so you know that it 'is' possible to roll your own.