I don't have much info myself, but I browsed around and "Trooper TK409" posted an old thread on the rpf, wherein Gonk size was estimated as 57" tall, 29" deep, and 18" wide. They said correctly sized bins were hard to find, and used 18 gallon bins (for a costume for a 7 year old, mind you).
Edit: Didn't you build one already? Or does therpf have their own completely different DeathProof that was doing a Gonk this year?
Addendum: There is a brief glimpse of the Gonk costume that I saw at Dragoncon @ 1:37 in this video. No idea who it belongs to.
It's also seen @ 2:32 with (if I'm not mistaken) shmorgan's R2.
Edit: Didn't you build one already? Or does therpf have their own completely different DeathProof that was doing a Gonk this year?
Addendum: There is a brief glimpse of the Gonk costume that I saw at Dragoncon @ 1:37 in this video. No idea who it belongs to.
It's also seen @ 2:32 with (if I'm not mistaken) shmorgan's R2.