Just to let you know the latest,
My experiments continue and I have become increasingly pleased with the results which I find I can now repeat.
I have tried various size bottles including 1, 1.25, 1.5 & 2Ltr
My favourites of choice are the mineral water Icelandic flat bottomed 1.5Ltr and the 1.25Ltr Coke.
The method is the same in all cases with a Schrader tyre valve fitted to the cap and the bottle inflated to approx. 60 psi.
I have tried shrinking one side first before expanding the other however it seems to be better to to the expansion first.
My trials have also revealed that it is also possible to significantly increase the size of the bottle round the neck area in a similar way that the base can be expanded.
1 & 1.litre will go in the box either way and the 1.5 & 2Ltr are best diagonal - unless you want to cut the neck and fit a ping pong ball in its place.
I have become increasingly impressed with the abilities of Zappa-dappa-Goo. This glue is No1 in my box closely followed by clear Gorilla glue which is also foam friendly.
The choice of heat source was to start with problematical, however I found the halogen heater to be just right and that the heat is focused.
Yesterday I found that running the hole saw back-wards in the drill prevented it snatching at the bottle. - This had been the cause of some splitting and un-necessary stress to the plastic. This is fine for round tubs but you will still need to cut a template if the tub is another shape.
After getting the bottle to the shape you like make sure you take the top off and allow the bottle to cool. Leave it on and you will come back to find your bottle has a crease that was not there before - YHBW
Using a soldering iron for making holes is also highly recommended as the hole becomes sealed at the edges and less likely to tearing than when drilled.
To give the Balsa/Ply/Balsa keel and balsa rudder some protection, I have used some finishing epoxy.
More another time