I am trying to size this thing for North American 2L straight-walled pop bottles. After many iterations and sizing changes I finally found the size that works, with the bottom of the thing slipping over the cut bottle bottom and the top fitting inside the bottle top. The inside fit doesn't seem the way to go, so I plan to change the top of the thing to match the bottom with an inside fit.

It is getting bigger... not what I had in mind initially, but I guess we have to go with what works.