You need the media basket because it directs the water from the intake to the bottom of the media chamber where the water then rises through the media and over the overflow into the tank. Without the basket water may or may not pass through the media, most likely it will take the path of least resistance and bypass any mechanical filtering media. I'd think you could find a replacement basket for a few dollars at a good pet shop though.
the media basket on AC filters is only half perforated, there's some solid areas and ridges along the bottom to encourage flow from the bottom up. You may be able to make your own basket, but I doubt the water would flow through the filter as intended without the original AquaClear basket.