I'm in the process of building a woodstore, a bit like this one:

The overall floor area is 10m x 3.5m. I'm using 10 foot long deer strainers for the posts, and had them driven into the ground last week by a fencing contractor who was doing work next door. However, since the strainer posts are round, I'm not sure how to go about attaching the horizontal beams at the back and front that will support the roof joists.
I'm probably going to use 2x6" or 2x8" timber for these beams, but should I:
a) Rest them on top of the posts
b) Bolt them to the side of the posts
c) Sandwich the posts between two 2x6s bolted through
d) cut an L-shape into the top of the post and do a hybrid of a) and b)
or e) Something else entirely?
What do you think folks?