The trick I always use is this: - With 2x4's, frame the opening that you want. - drywall the wall around the hole, bringing the drywall half way on to the studs on all sides of the opening so that approx. 3/4" of the stud is still showing. - cover the edges of the drywall with metal or plastic "j channel" to hide the rough edges and to make a nice neat opening, don't nail in place yet. - Cut a smooth piece of plywood to fit into the opening with at least 1/8" gap all around. Sand smooth and paint. - attach the plywood panel to the studs with velcro or with screws dressed up with finish washers. If you use velcro then you might need to make a little tool with a wire coat hanger to get into the gap and pull it off, or just use cabinet pulls. - nail or glue the j channel in place so that it leaves a nice even gap all the way around the panel.