Finally ditched the trapezoid subwoofer special and put the Audio Integrations box in. Bumped internal volume up from just under a half cu ft to 0.65 and think this sub is a little happier there. Also wired the two 4 ohm voice coils in parallel rather than series (NX5 is 400w@2ohms, versus 200w@4ohms so 100w@8ohms?) and seem to have a ton more headroom!
Obligatory pics (yes, I was too lazy to vacuum):