Here's something you probably don't know about me ... I don't have a sliding miter saw. Here are some other things you probably already guessed ... I do have a regular miter saw; I don't really have the space for a sliding miter saw, let alone a second miter saw; I rarely have the need for a sliding miter saw, and I sure the hell don't want to pay for a sliding miter saw.
Looks like it's time to make a solution ... and then fix the fence on my large table saw sled.
Step 1: The Parts and Layout
You've probably seen a track saw, and they are awesome ... awesome at being expensive. You've probably also seen a saw board. In fact, I was making a conventional 4' one when I thought to myself, "Self, You need to make me a sandwich. But after that, you need to modify this design so we can cut up that sweet 12" wide board that the miter saw can't handle."
Sorry, I got distracted. We'll focus on the modification. 1/2" plywood which I cut down to 10 1/4" x 18". A strip of 3/16" hardboard cut to 1 3/4" wide (left it long). and another piece of 1/2" ply that was 1" wide and around 10 1/2" long (left it long).
The offset to the left of my circular saw blade is 5" so I scored a shallow line at 5 1/8" on a table saw. I'd recommend going a little more then the offset for a cleaner first cut (I know I should've gone with 5 1/4"). This shallow line is an alignment line and a trick I found on youtube.
Step 2: Glue and Screw the Rail
Slap on some glue, line up your rail with the shallow cut, and throw a screw in one end. Now dial in the other end and make sure you use a square for verification. Drive some more screws. I used # 6 1/2" wood screws and I predrilled the hardboard with countersinks. I trimmed off the excess hardboard at this time.