Is anything squirrel proof!!
I think if you want "reasonably squirrel proof" concrete or resin or kiln fired clay is the way to go. Foamex (is it called Sintra in the USA? ) is reasonably weather proof and light (easy to put away). you can paint it with weather resistant paint of your choice and add marine varnish if you want. You may want to construct it so its easy to add weight with rocks, bricks etc. You may want to fill or close it off so critters can't nest in it. The main advantage of Foamex is its versatility. You can score bricks and planks into it, cut it with a craft knife, paint it, or get your designs commercially printed onto it, or print yourself onto paper, card or adhesive vinyl film. This means you have complete creative control over what you produce.
I don't know what would happen to home or commercially printed Foamex if you left it in strong sunlight for long periods. Distortion or colours fading? My layouts are usually built outdoors in fine weather but displayed indoors. A few years ago I built a WW1 display model for a Safari lodge in Kenya where my friend has created a WW1 museum about the fighting in the Tsavo West area.. The Foamex baseboards started to distort due to the intense sunlight in one of the hottest summers we have experienced so I rigged up and awning. the display is located in a cool shaded part of the lodge.