For light duty frames you can use shot-in stays where the upper end is integrated with the seatpost clamp.
The classic method is good for light-med touring. There was a neat weight saving trick (shown by Peter White if I recall) using a reversed section of tubing (concave-out) cut form a scalloped end rather than a heavy investment cast plug. This example does it without the scalloped shape.
Heavy duty frames often extend the area of braze by using wrap-around plugs.