A wooden or laminate surface of the proper siize needs to be purchased from a woodworker or a supplier such as Home Depot, Lowes or other lumber supply. The contact side should be lined with felt or other fabric to prevent damage to the pool table. The weight alone is sufficent to kepp it in place after laying it across the pool table.
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While you could convert a dining room table to a pool table, it would be too high to be considered a regulation table, and may not even be comfortable to play on. It certainly would not be a good practice table.
More likely, if you have limited room, is to use a pool table and convert it, by placing the proper top on it, into a dining room table. These are sold commercially and can be found on the internet.