Where to Find a Rubber Flush Mount Plug for a 1/2" Hole?

I need to seal a plastic container that will be filled with water at the bottom of the container. In order to set down the filled container onto flat surfaces, I need a low-profile flush-mount rubber plug. For the same reason, a rubber plug would extrude too much to keep the container flat. Here is what a rubber flush mount plug looks like for a 3/4" hole. On Amazon, I find a lot of these in plastic (i.e., polyethylene). I need rubber. Does anyone know of a good supplier for the part I need?

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