When I started cutting boards for my latest build, I ran into the issue of the sheets of MDF not being square right from the store (lowes). Frustrated and determined, I struggled along with my track saw and my table saw but soon realized I was just wasting material and time trying to get pieces of 32" x 15" squared up. I have the Incra 3000 miter gauge, which is awesome, but it does not allow for squaring up any larger width boards than about 8-10 inches. I have been meaning to build a nice table saw sled for a long time, and this was finally the situation that pushed me into taking the time (and money) to do it right.
I started off with a nice sheet of Baltic Birch. I believe the final dimensions are 30" wide and 24" deep. It will accommodate up to a 22" width board. I glued up two layers of BB for the user-side fence on the sled and two glued up pieces of Maple for the front side fence. I purchased Incra's fancy T-slot sliders which are 25.5" long and also a 48" piece of Incra's T-track so that I could add hold downs to the sled. I also threw in a set of Incra's Built-it Hold Down Clamps for use in that t-track.
Over a couple of days of wrapping my brain around the squaring up process, I was successful using the 5 cut method of table saw sled adjusting and ended up getting it within 0.00075" over about a 70" total length. If you don't know what I'm talking about, look up the Wood Whisperer's table saw sled build video for more explanation. Basically, after the adjustments were complete, the sled is ridiculously accurate.
After a week of downtime from the current speaker build, I will finally be able to get back at it tonight and avoid all of the previous frustrations about panels that are not square!