Stuart,
I don't secure my logs at all. I find it a wasted effort.
Instead, I have two large logs on the ground with saddle-notches cut into them, kind of like you would use for a round log cabin. I then roll the log to be hewn into these notches, and it will set there securely if it has any size.
My procedure is then to hew down one side, and flip that side to the bottom and move it around until this face sits level (or rather, square to plumb). The flat surface will be sturdy, even more so if it is made to straddle the notches giving it 4 points of contact instead of 2.
Where are you located? If you would be willing to come out here to Indiana, I could help you out. Or I could send you some info on the use of these tools. There are a lot of particulars that are hard to figure out on your own.
