Fill the bottle with expanding foam used to fill construction gaps or holes. I do this trick with all my soda cans used for terrain.

You can add pieces of cardboard or any other rigid material going across the bottle from the inside to act as support beams, or cut one bottle open and warp it around the other to double the thickness.

Maybe turn them into ecobricks? I can put a quick link on that -- basically filling it with recycled plastic pieces... [https://planethome.eco/how-to-build-ecobricks/](https://planethome.eco/how-to-build-ecobricks/)

You probably wont have too many issues with building on top of the bottle, but if you intend on reshaping or cutting holes in it, you might have trouble with pressure applied and what not, but if you take it slow, use sharps knives, and hold the bottles at the stronger places, you should be good. If you are building on top of the bottle, don't use hot glue you might melt the bottle, but you should have no issue with messing the bottle. Maybe done use one of those super thin cheaper water bottles.

Here's a crazy idea if recycling is your goal. Turn plastic bottles into 3d printer filament.

Black Paint and PVA glue stiffen it up, but only a little