Update 2/21/2009 -
Check out the pictures of this project.
Check out the next generation of this sled.
This sled has been good to me, but I find myself using Wood's universal tablesaw sled more and more lately. My sled has a 1/2 thick inch bottom that I made by laminating two pieces of hardboard. It was stuff I had laying around, but it was sort of melamine coated on one side and rough on the other. I thought the melamine side would be less friction, so that side is down in contact with the tablesaw top. I though the rougher side would be good to hold the workpiece a bit.
Here's the quick summary of how I went about constructing it. I made some material for the runners (plane it down until is fits perfect (even a little tight perhaps). I used oak, but if I had it to do over, I'd use the UHMV plastic material. Place the runners in the miter slots and then put on a couple of pieces of double stick tape on top of the runners (or on the bottom of the sled). Without contacting the tape just yet, position the sled's base onto the runners. Now contact the tape to stick the runners to the bottom of the sled. Hopefully you can get the runners out and still have them stuck to the bottom of the sled. I've seen other methods - like super glue and brad nails from above. The point is, you want those runners to be positioned where they should be and then flip the whole thing upside down and secure them for good (I used countersunk screws in the bottom).
After your runners are secured to the base and sliding well, it's time to attach the tall backer board. To get the backer board square, you position your sled's base and raise the running blade so that it cuts through your new sled. Bring the blade up in the middle of the sled. That is, don't cut all the way through the front or rear of the sled base. At this point you can take a framing square against your blade and draw a line square to the blade where your backer board will mount to.
I made my backer board from left over mahogany that I had, but any straight flat hardwood would do. The t-track modifications I made to the backer board have been terrific. There's no substitute to having stop blocks for repeatible cuts.