The sled we use at the gym is simply a normal sled with a rubber gym mat bolted to it, cut into the sled dimension of course. The nice thing is that you don't need a ton of weight because when you add weight the rubber causes extra friction... also it's never damaged the floor.
I personally bought a sled off Kijiji for 20$ it's a good place to check. If I didn't find one I was going to make one. Here's some ideas:
http://img42.imagevenue.com/img.php?...5_PICT0416.JPG
How to make a Tire Sled:
http://forum.bodybuilding.com/showth...7223533&page=1
I was also thinking of using a toboggan and just adding loading pins with plumbing pipes and flanges. All very cheap if you get the sleigh from Kijiji.
As far as the harness goes, I have a real harness that I don't really like, it chokes you. A chain from the hardware store you loop into a loose weight belt is my preference. Get some carabiners to attach it to your sled.