Paul...
My picture is a almost 100% crop of the 10D + 100-400 @ 400mm (same as your gear). So the moon is actually a very small circle in a very large black frame.

Quality isn't great, but it's fun!
Just think of what Mark's 600 + 2x does!

I can imagine THAT creates some substance in the picture...
I shot in M-mode, 1/60s f7.1, but I could have done a bit longer to make it brighter (the moon wasn't at its brightest yet). I focused with
AF once, then switched it to
MF and left it there. It's actually quite amazing to see how fast the moon moves. After a few minutes you need to re-adjust your tripod...
Eduard