I've never used this type of material for that purpose, and I dont know about its optical properties. Applying general priciples though, you should utilize the center of the fresnel sheet. The center should be the "sweet spot" as the fresnel rings converge there. If you cut a small piece from the edges, it may only direct light in one direction, or disperse the light in a scattered pattern. To create heat, the light should be focused from all directions to the lens' focal point.

Experiment yourself or maybe someone else has some good practical experience with this. Good luck.