Based on the project (chicken coup) and the very coarse drawing I think a reciprocating saw would be more appropriate and safer if you really need to cut it in place after installation. (But why?)
In the picture I think you have the top beam extending out over its support and you are cutting it to length. As pictured it could pinch the saw as you cut it from the bottom up and then send the spinning saw towards your body.
You need a flat surface to hold the circular saw against and guide it. If the coup is sturdy you could use its frame for that purpose, once fully built.
A better approach would be to cut the pieces to length on a more stable surface before assembling. IT doesn't have to be fancy. You can place a piece on the ground protruding out over the edge of a stair or ledge, and stand on it. Then the saw is guided by the piece itself, you're cutting from the top, and your body is above and aside from the line of cut.