I would get a piece of foam, 3" thick. You could glue it down to the wood with spray glue, or just leave it. Take a sheet or a piece of muslin and staple it down on the underneath side of the top. You could remove the hinges and mark the muslin with a pencil to indicate the screw holes (for later use when you reattach the hinges). Pull the muslin over the sides to round the sides of the foam. Next, take your upholstery piece, or whatever you are using for the top, and staple that over the muslin, again marking the holes for the hinges. You have several options for the underneath of the top. You could use an upholstery trim and glue it or staple it over the edges of the fabric to cover it; or,you could take the upholstery fabric and cover the entire piece, but adding trim or velcro to finish off the edges of the fabric. You could open the velcro and slip secret or important papers inside. That is an option. If you are using a vinyl or leather, you could finish the edges off in nail heads, your choice of color. Hope this helps. I have done this before, and I have had no trouble. If you need help, please contact me.