Looks good. Though you would get better heat retention using thick wood, or 15mm glass (less so with glass), than the sheeting you use, and combined with lighting. A cheap alternative insulation is Styrofoam sheeting (1 inch).
What I fear you will find with this is that the heater will have to work hard, not heat effectively, and you will use more power to achieve the result you want. If you went with wood, a good LED (you can get some really powerful ones for planted thanks that have the right kelvin range at aquarium shops), and a heater it will be more energy efficient and stay warmer.
A passive heat source like a big vessel of water with a large aquarium heater will only heat to 40c from memory, and will not give you much boost to ambient temps.. I read that they only boost temps a few degrees over the outside temps. Its rather similar in effect to the greenhouse tube heaters that you can buy.
Brett
