I’ve got Home Depot’s and Lowe’s easily accessible. They have these two options at Home Depot.If you have an industrial surplus store in the area check out high density foam sheets.
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They come in varying thicknesses, cut easily so you can cut them down to height of the pictured concrete and will adhere easily with any construction adhesive. If you don't have a surplus store then find a wholesaler if possible. Fabric shops up charge them ridiculously high.

FUTURE FOAM 3 inch thick High Density Blue Swirl or Copper Swirl Memory Foam 10030MEMORY3 - The Home Depot
Multi-purpose High Density Bulk Memory Foam is the superior choice for the do-it-yourselfer in need of a high quality product for cushioning. This High Density Memory Foam is manufactured to hold its resilience yet be easy to cut, making it the ideal solution for a wide variety of applications...
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3/4 in. x 1.25 ft. x 4 ft. R-2.65 Polystyrene Panel Insulation Sheathing (6-Pack) 150705 - The Home Depot
Provide your home with effective insulation with the Cellofoam 3/4 in. x 14-1/2 in. x 4 ft. Polystyrene Foam Insulation Board. Ideal for both interior and exterior use. This durable insulation features
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