I came up with this slick way to drill a large, straight hole in the end of a post. It works great and makes good use of my tall floor-mounted drill press.
To make the jig, I first swung my metal drill table to the side and clamped on an auxiliary table. I made the table out of two 3/4-in. thick by 16-in. wide by 32-in.-long boards, glued and screwed together. The double thickness is required for strength because the drilling action puts a lot of pressure on the table. Then I built the jig and screwed it to the auxiliary table. The 16-in.-long side support board keeps the workpiece vertical in the left-to-right position. The front support board keeps the workpiece vertical in the front-to-back position.
To drill a hole with this setup, I clamp my workpiece to the jig. Then I loosen the clamps holding the auxiliary table in place and adjust the whole setup until the drill bit is correctly positioned over the center mark. If the jig setup flexes while drilling, add some additional support under the bottom of the workpiece, between it and the base of the drill press.