Bought the Academy/Trumpeter T-69-II on a whim (no aftermarket resin/ PE/tracks for this, just an OOB) but the lower hull is badly warped/twisted ( the front/rear are both about 10 degrees from horizontal). The upper hull is so thin that simply attaching it to the lower will result in the whole thing warped. I figure: rubber-band the piece flat to a chunk of balsa and dunk the whole she-bang into hot water. Question: How hot the water and for how long? Suggestions greatly appreciated.

I used the hot water fix for my ICM Pz II D. It wasn't warped as badly as yours according to your description, but it should work for you just the same.
I was cautious at first, using water from the tap, at full hot. Not hot enough.
Then I tried heating it in the microwave in a coffee mug for about a minute and a half. That did the trick. I know microwaves vary like any oven, but the gist of it is I used water that was not boiling, but pretty hot. Not too hot to stick your hand in, just not leave it in there too long. I guess I left the part in there for around 8-10 seconds.
Clear as mud? Good.
I'm sure different plastic could react differently to the same temperatures, but anything below boiling I think would be OK. If you're worried, do like I did and work your way up until it works.
One more thing, I found that after the hot water bath and de-warping, a rinse of cold water helped set it to it's new shape quickly.

I have used the hot water method on plastic and resin kits. The cold water rinse does set the part after taking it out of the hot water.

Howdy Zoomie,

Whenever I have a warpage problem I just give Harry Potter a call at Hogwart's!

Actually the hot water method is the one to use. The way that has always worked for me is to simply boil the water in a pan. Just as soon as it begins to boil, remove the pan from the burner and put it on another burner that has NOT been on. Wait a very short time (approx. 30 seconds) and then pour the water into another cold pan.....at this point you can instantly place your piece into the water and perform your 'task'. The water at this point is just hot enough to work, but cooled enuf to be bearly tolerable to your hands for a short period. Try it. It works.
The cold water in another pan is great for 'fooling' the plastics memory.

Tread.

Additionally.....DO NOT place kit supplied 'rubber band' tracks in the microwave to soften them up for possible sagging, and/or stretching...................silly idea really.

Thanks, Guys! Will give it a shot this weekend. Regards to all, Rob