... I have tried the same technique with a Freznel lens and was dissapointed with the result. Doesn't seem to focus the beam quite as precisely as glass.
Most Fresnel lenses are floppy plastic and optically poor. At the best of times the focus isn't so good as even a modest quality glass lens, but when they flex, the focus goes all over the place. If you can mount the lens on something that will keep it flat that will help a lot. The nice thing about them is that you can get very big ones quite cheaply, and the large size can more than make up for the slightly dodgy focus. I have one about 8" x 10" (cost a couple of pounds) and in strong sunlight, even in England, it will light a crumpled cardboard box in five seconds flat.
It's worth preparing the area where you will focus the beam. Something like some dark tinder nestling in the inside corner of a cut-open cardboard box is good. The sides of the box heat up and so reflect heat back to the tinder in the hot spot that you're making. That can improve the energy density no end. Even if the tinder is damp you can dry it out very quickly using the lens, and then it will light easily. Dark surfaces absorb the sunlight better than light ones, so if you have for example paper which has been printed with black ink that's usually better than white paper. The tinder material should ideally be fibrous to allow oxygen to get at it better when it starts to burn.
