After about a 45 minutes test, the interior of the lunch sack was 50* F warmer than the chamber's air temperature (-10 vs -60). And this was with one sheet of foam rubber missing and no power applied to the tracker. I did notice the antenna elements did get stiff. But they are silicon jacketed 14 gauge stranded wire, so it should be just fine.
I need to look up some information on Rocketman parachutes to finish this article for Popular Mechanics online.