Concentrating sunlight by any means does not make heating any more efficient. Infact, efficiency is generally inversely proportional to the reciever temperature. Concentration is used when high temperatures are required - such as for steam generation. The energy available from the sun is nominally 1000w/m2 of collector area directly facing the sun, and is typically less if the collector is 'off axis', the sun is obscured or low in the sky. For most domestic heating purposes, a simple collector with modest insulation will provide sufficient temperature without the need for concentration. You are much better off expending your efforts/money on increasing collector area. For example, the 8"x11" plastic fresnel lens you mention would provide approx 45w max to the focal point if correctly aligned to the noon day sun (this would be its absolute max output). If you could retain all this heat (you can't) it would take approx 13.5 minutes to boil the water in one glass of water. Not only would you need a lot of lenses to provide any usable amount of heat, you wouldn't capture any more energy than a similar size flat plate collector. The flat plate collector would also have the benefit of performing much better when not facing the sun directly.
I have made approx 170ft^2 of collector area using aluminium flat plate in a simple frame with urethane foam insulation and glass covering. The all up cost was approx US$700. This provides 99% of all the energy required to heat my 2800ft^2 home (water included).
