Hot tub cover
A hot tub can be built by just about anybody with the right knowledge.
Hot tub or jet spa
While some prefer a wooden hot tub that can be heated by the sun and will give you the feel of a hot spring others prefer the jet spa experience. A tub heated by the sun may be cheap but can only be used in summer time.
Materials
For a hot tub, you will need poly vinyl tubes to make the hot tub, or just a half barrel or tub. Use a solar pool cover. It will attract solar heat if placed in direct sunlight.