If you can: do not use cardboard

kicsikato

thanks, it was useful!

TahaLea

thanks for the tip

oysterCracker

never thought of this, good thinking

kvetka

The only place I use cardboard is as a purse bottoms. I wrap it in fabric in a color to match the color of the yarn. I place them inside the purse, to make the bottom sturdy and flat. They are fully removable, before any wash. I never had any problems with them. They last in my purses for decades!  Shampoo bottles, and milk jugs are round, not flat and not wide enough. They are not suitable for bag bottoms. Plastic is not environmentally friendly. Other than that I don't use carboard in any other projects.

allegra

I use plastic canvas.

Crochetlover22

I use plastic canvas and sometimes those thin plastic cutting boards from dollar tree