I had a Blizzard1230 Rev3 and I ran it with the trapdoor on without any problems. I guess it's not exactly recomended but it did work. If you want to leave the trapdoor of you could open the machine, and to the right of the card there's space enough to screw on a plasticcard that will stick out the left where the trapdoor "lock" sitts. I did this with my old BPPC equipped A1200. Use a semisoft card or other plastic and don't have it stick out too long over the CPU... this so you later can remove the card without haveing to open the machine and remove this "fix". Another way is to simply cut a hole in the trapdoor over the CPU or create your own trapdoor (I made one from metallmesh of a speaker... it's easy to modify to your need, give some protection and have great airflow).
