Wow - I can't believe how many memories this brings back. That damned original dark-screen GBA and the myriad solutions for viewing the dim screen.
That being said, the worm light was pretty useless, because it brought a pretty bad glare to the screen. Back in the day, the best (and, at the time, only) solution was to either buy a backlight kit like the Afterburner or to send it in to one of the people who installed the backlights.
Nowadays, it's nearly as inexpensive to pick up a used GBA SP or micro as it is to buy an external light, and I'd recommend that. But if price is that much of a factor, a desk lamp will probably work better for you.