Suitable surface to cover old harvest table for use as desk
You can buy plastic deskmats. For example, I have some that are about 2-foot by three-foot. These are suitable for writing on unless the underlying surface is extremely uneven. The old-fashioned alternative was leather or blotting paper (in a "desk-blotter").

Otherwise I'd use a sheet of attractive wood of some kind, planed flat and edge-jointed to whatever size seems appropriate. Finished to taste (e.g. varnish). You'd still cover it for writing.

The cheapest is a pad of paper (e.g. 50 A4/Letter sheets with a cardboard back).