Mary,

This idea is so simple that chances are that someone already wrote about it here (and elsewhere) - please forgive me if this is the case.

It is also not original, not fancy and not the lightest setup - but it works.

For additional safety, tie the bottles neck to neck beforehands. Tying will hold them together, so they aren't lost if the hull is cut in half. And, should it happen, the string of bottles makes for a good flotation aid.

I also learned the following in the discussion site br.groups.yahoo.com/group/altomar/messages (not in English):

The empty bottles are tough because they were designed to take relatively high internal pressures. Participants reported on pets being used as fenders (without cover) and mooring buoys as well.

When higher internal pressure is desired, they put them into the freezer before closing the lid. The cold air then expands, increasing the pressure. Depending on where you are, just leave them outside at night and close the lid in the morning.

Take care!