The color of the material gives you a very good sense of how good it is at absorbing heat. Specifically, if the material is black, it is not reflecting any (visible) light and all of that light is being absorbed. You could do this just by coating the surface with black paint. (You can do even better if you find material that also absorbs non-visible wavelengths of sunlight, mostly ultraviolet and infrared light.) But keep in mind that the best you can do is absorb all the sunlight that hits the surface--that means having more surface to work with can be more important than a 5% increase to your absorption spectrum. This is why reflective surfaces that can send more light to an absorbing surface are usually so important to solar cookers.

The thermal paste situation is interesting. Without using commercial-quality materials, toothpaste and Vaseline seems like a fine substitute. But I'm not clear on why you need it for a solar cooker. The purpose of thermal paste is to improve heat conduction from one surface to another (like from the light-absorbing surface to something you want to heat). I would guess that you could accomplish this just by having the light-absorbing surface as a wall of the solar cooker.

That being said, I'm not an expert on solar cookers at all--I would not be surprised to find out that you already know all of this. Just in case, I want to make sure you leverage as many resources as you can: read about building solar cookers using a search engine. For example, I found some of the basics on this site . They mention a strategy called glazing that might help you: by putting your entire solar cooker in a transparent box or bag, you can trap some additional heat. This works similarly to a greenhouse, where heat has an easier time entering than leaving.