Well, first of all sink some massive posts...sorry, I couldn't resist. You don't need much here as you are only going up 3-4' (right?). You could use a pressure treated 2x4 for the post, set the lattice around it and cap it all with another 2x4 around the top. That will strengthen all of it.

The biggest thing here is to not impede the air flow to the condensing unit. Take a 3x3 section of the lattice material. That is 9 sq ft gross. Now figure out the "free air" opening of the lattice. I bet it isn't over 30-40% of the gross opening. If you are going to do this, put the surround at least 2' away from the sides of the unit. You affect air flow, you affect comfort and efficiency.