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Pvc is the most realistic priced option. Whether sheet or small tubes . Tubes are prolly better for slideability/availability. Parks generally use HDPE but it's expensive and imo not worth it generally for backyard.
Build a box out of 2 x 4's(wood) cover it with plywood and then line the outside with plastic sheeting. You can also build one out of PVC tubing, get all your fittings and glue it together. Either way, should get you a decent set up.
Real answer: get a nice sized PVC tube (4-6" by 10 or more ft long), screw a 1x4 board under it to make it stiff. Or get 2-4 smaller tubes (1-2") and get a 2x6 board to screw and you can make a flat topped thing easily. I like to make pillars to attach to the underboards and make a nice wide base for stability