I've been busy reading into Yagi antenna design, and have come up with a few designs for the Yagi's we'll use to track the balloon in flight. I'll be building the antennas from aluminium, to keep weight down. I'm planning on a 1.5m long beam, allowing me 10 elements, and a gain of ~14dBi.
I made up a small 4 element yagi just now, with about $2 of materials - some wood, a few coat hangers, PVC pipe & zip ties. Haven't had a chance to test it properly, but it seems promising. Modeling it in Yagimax suggests a gain of around 9dBi, which is great given the cost & time invested in it. Hopefully will get a chance to make up a long yagi in the next few days as well as test it out.