I've never seen such an item for sale, but it would not be hard to get something like this made. Any competent woodworker could make the base. If the base were made with a recessed 1/4" or 1/8" (depending on the size of your case) deep rabbet on the top edge, a plastic shop could make the top acrylic piece. I imagine any of the shops advertising turntable dust covers could do this. As to what's a "reasonable" cost, that's subjective. I imagine it would cost at least a couple hundred bucks to get something like this custom made. Although you've got a good idea, I can't imagine there's enough demand for this to warrant manufacturing this on a large scale.