The only issue with building a sled is finding somewhere to buy a small piece of steel plate. Buying a full sheet of steel plate is a little bit much to build one sled. If you can find a small welding shop nearby you might be able to get them to build you one at a reasonable price.
All you really need is a piece of plate with a piece of 2" pipe welded onto it, and a ring to connect your harness to. I would look around your local area for some welding shops and see if you can get a better price. It will probably depend mostly on the shop and how much they want to charge for labor.
I have had this thought, but my lifting partner found a used one on ebay. A used one might be the cheapest option. I know it was the cheapest option for me.