OK, I've just checked and the
photos on Nacker's website shows the mount bungeed up as it should be; except that he's cobbled it together with zip-ties
The bungee goes up from the back, through the outer holes and back down through the inner holes. The ends are then tied together.
For a more professional finish, just use a fisherman's knot in place of the zip-ties and the bungee will lie in a channel between the two inner holes.
EDIT: Just spotted that he uses just one length of bungee crossed at the back, which is all well and good, but if the bungee breaks you could lose the computer/timer. So I'd recommend using two lengths (one either side) for redundancy. To be fair, he refers to this in the text.
The photo for the
Suunto mount shows two bungees but with stopper knots.