I also had the need to improve cooling of my A1200's underside when cramming in a PPC accelerator. I didn't want to leave the cover out altogether, so I cut some slits to help airflow. I also replaced the rubber feet on the corners of the A1200 with some higher ones, so air can move around more freely under the case. Here is no longer a picture. There is also a 60mm fan inside the case circulating air, plus the PPC's own little fan.
I'm still not very happy with the PPC's temperature, but it was rather hot while in a tower case too and seems to have suffered no ill effects. I don't think I'll leave the desktop on for long periods of time though.
