In fact, if you look closely at the photo of the antenna that Glen posted, you can see
that it uses the WA5VJB feed method, and the element spacing is typical of his designs
as well. So I'm guessing that is where the original design came from.
I've build a whole lot of antennas for transmitter hunting, including some yagis (and quads)
for 70cm, but the yagis are based on the WA5VJB designs (with some tweeking to center
the passband where I want it, etc.) So I'd just be repeating what has been said before.
The last antenna used PEX pipe (which is light), and I've also used thin-wall PVC. I rotate
the driven element 90 degrees about its axis so the fed end of the driven element is in front
of the main part rather than underneath it, allowing it to also pass through holes drilled in
the pipe. I then cut a notch in the top, solder the coax directly to the driven element, and
fill the notch with hot melt glue. The coax runs out the back end of the pipe, with a tie wrap
for strain relief.
I typically use #12 solid copper wire for the driven element and #8 aluminum ("ground wire"
from Radio Shack) for the parasitic elements to save weight, but on 70 cm that isn't much of
a problem, as the antenna is relatively short and light.
Somewhere around 4 - 7 elements is probably a good size.
You will find that, once you get close to the transmitter, it will saturate the receiver and you
won't be able to take a good bearing without some sort of attenuator. Hopefully at that point
you can triangulate and get a good estimate of where it actually is.
Oh, and remember to confirm that the transmitter is on before launching. One of the
local universities lost a balloon due to that minor issue...