A "homemade" IR filter is made from any old floppy disk (who still uses them?): you open it, take the disk, and cut it. But it's generally used on video cameras to be able to detect the position of IR leds (a few years ago, a head tracker for Flight Simulator worked that way). But I don't know if a normal light sensor would be sensitive enough to IR frequencies (I don't think so).
So I agree with @Wawa, but there still needs to be some testing to verify it can detect a flame.