I'd hate to take a chance on it peeling up the paint if I were you. I have used thin cardboard (like chipboard) before for stencils. (Like the back of a large desk calendar).

How about poster board? Or if you could find a really cheap plastic cutting board, the kind that is paper thin and you can roll up, check at the dollar store. Hope you post pics because it sounds gorgeous!

I used thin clear plastic sheet when I made border around 1 room. I have 2 stencil books handy & 1 has cardboard stencils in it that are coated so shiny. Other book by Plaid suggests clear cut stencil blanks I think they are about same size as sheet of typing paper. By using clear stencils you can see where you are working & have been so your design will flow. Be sure to use patio paints or outdoor paints.Spouncers(I call them pouncers) or a stencil brush works well. It is very stiff comes in several sizes so larger one for main part of flowers & leaves & smaller for shading. It's probably easiest way to do it. BE sure to have supply of paper towels as you want to get paint on brush & then pounce lightly on paper towel to even out paint for a nice project.Pouncers or stiff brushes go straight down in paint, paint should not be all over the brush just on the bottom end of bristles & you move it straight up & down on table on your stencil(never sideways-will just smear) Sounds like a fun & beautiful project! When finished I would spray it with 2-3 coats of Krylon matte, satin or shiny which ever finish you like best. Have fun. Jan