I know there are many excellent wood workers here, so this might be irrelevant for many of you. However, I just learned a new trick for us "box builders" (as opposed to cabinetmakers) that I thought I'd pass along in case it might be helpful.

For years, I've used a clamp-on aluminum straight-edge when cutting plywood or MDF panels. Mine is like this one: http://www.fleetfarm.com/catalog/product_detail/tools/power-hand-tools/power-tool-accessories/empire-pro-s-edge-cutting-guide?utm_source=googleps&utm_medium=shopping%2Bse arch&utm_campaign=google%2Bproduct%20search

It works well, but the one problem with it is that you're always putting the circular saw next to the straight edge instead of on top of it. This results in you having to factor in the space between the edge of the saw's base plate and the blade when measuring for a cut. While that might be easy for some, I found it to be tedious and made cutting many panels more laborious.
I heard about a DIY straight edge that was easy to make, inexpensive, and worked really well. Here's a video of it on YouTube: http://www.youtube.com/watch?v=CH5dW-QcgeI

For around $10, I made two of them yesterday (4 and 8 feet long) and they worked GREAT!!! You make the marks where you want the cuts to be, clamp the edge on right at the mark, lay your saw on top of it and cut away! I cut enough MDF panels yesterday to build two 12 cubic foot subwoofer boxes and it took me about 1/2 as long as it had in the past. In addition, each and every edge on the panels I cut are more smooth and just as straight as the factory edges on the MDF.

The only caveat is that when you make one of these, it will only work for that circular saw and, I assume, the blade that was used to make it. I'll mark my panels with a Sharpie so I remember which saw and blade was used when making them.

I'll post pictures of the subs once they're built. For now, I had to pass this along because I am so pleased with how well it works.