Canon cameras don't tend to be very IR sensitive with their IR filter in place, a while ago I pointed my 7D at an array of IR LEDs and you could barely tell they were on. By comparison pointing a monochrome CCD camera without an IR filter at the same array of LEDs totally saturated the sensor.
If you were after a cheap DIY solution you could try a cross-type red laser module, but for safety / legal reasons it wouldn't be any good if the subjects are people or animals. Otherwise try a torch with a cross or cross-hatch cut out of thick cardboard and see how it goes. I think the only purpose of the pattern is to give a few lines with sharp contrast, I doubt it has to be all that precise and the IR assist on my 580ex looks a bit 'fuzzy'.
Edit - just realised after posting the IR filter is on the main sensor not the AF sensor, so maybe AF would work OK with IR. They might just stick with near IR so you can see it's working.
