Thanks Countryboy and Bill. I'd do just what you recommend, but the amount of the rear herex foam bulkhead seems too meager to do it. Don't know if your Roo's were of the same type but on mine, compared to some, the hole leaves about 1/4" of material at the top and the bottom of the former and beyond that I'm right at the skin of the fuselage. I think I would have to remove all of the material to fit the standard 2 liter bottle and I'm concerned that since the foam is so weak I'd have a problem with bulkhead strength.
If I'm not mistaken, it seems that the typical 2 liter bottles now used for soft drinks are shaped with a longer neck than those I've seem used before and that seems to extend it too much for this application. Hey, I could be worrying more than neccessary about it but those of you who know --clue me in.

John