Nice write-up on building a straight edge guide for your circular saw. This is on my todo list but…I am thinking of upgrading my saw first. As the author points out. you could most likely get HomeDepot to make the cuts you need for you. So you only need to nail and put on the finishing touches. (maybe add a handle too).
http://www.instructables.com/id/Perfect-Cuts-With-A-Circular-Saw-Guide/