I made a base for my mount out of 2 x 4 material that was ripped to 3 inches wide. See photo. The height of the base is 20 centimeters. That's exactly what you need to work in a comfortable chair. I assembled it with wood glue and drywall screws and covered it with a coat of polyurethane varnish. I used some heavy duty sheet metal screws to attach the bracket to the base. That works great.
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