Plastic is worse than wood at shrinking and swelling, Sun hitting that side causes it to draw up just like wood. If you secure it to anything rigid it needs to allowed to move within the fastener.
Thinner plastic MAY be a little easier to control, but it will still move.
Personally I use steel and or cement. A six dollar bag of type S mortar mix works easy as a permanent base, but it is permanent. I have a couple structures that I have set in the mortar mix before it sets up,
This allows a base where no weeds or mud from rain shows up and you can texture, stones, and wood planks in the surface. Nice to smooth off and place a few cracks in the surface.
I pour my cement in the evening then texture it early the next morning so some 10 hours after it has been poured, do not try to texture right after it has been poured, let mostly dry first, but not cured.
I also use a 16 gauge metal for small structure bases so I can fasten details and people to the base, so when i pick up the structure the details comes with it.
Have fun
Dennis