Drawings look good Meru, however I think the 110mm between top and adaptor plate is a bit much. My pier has that dimension at 50mm. Just enough needed to get your hand in as you said.
Also think about using the 4 foot lengths of threaded rod in the footings. The rods are held in place by plate steel every 18 inches or so. so you have a sort of box kite frame that the concrete is poured around.
Good idea to orientate to true north as well.
http://aa.usno.navy.mil/data/docs/AltAz.php
This site helped to get the suns azimuth at any given time, so using the shadow cast on the adaptor plate of a centered pencil, reasonably accurate true north orientation is quickly achieved.
John