I previously used one from Accessible Systems, when I was doing some extensive work on a Triumph GT6. At that time (roughly ten years ago), it cost about $900.00. It was the version with the hydraulic jacks (recommended).
Considering the cost of materials and the labor required, I would not waste my time trying to build one and would just get the store-bought model.
Remember, your safety is paramount, so please don't take any risks to save a few hundred dollars.