The Reptile Basics panel mounts with two screws. You'll probably have to put a piece of wood on top of the lid so you have something to screw into and keep the top from sagging, but it's very simple. The 40-watt panel would probably be plenty, although since the price isn't that different it might be worth getting the 80-watt anyway if you think you'll move to a much bigger enclosure later. As long as it's on a thermostat it won't overheat, so as long as the lid is big enough for it, there's really no downside to getting a bigger one.

Just make sure you don't set the panel down on its face when you take the lid off!