I have to wonder if some of these leaky door issues aren't about the door, but the air. I just got mine last week and it was nice and tight. I ran my probe wires through the upper corner of the door and still only have a little bit of smoke leaking under certain conditions.
If you have the vents all set where air can get in and get out through the top, you shouldn't have a leak problem that's a concern. With the 18 only having the 1, 3 hole vent on top, I can't see any reason to ever have it closed at all. With all 3 wide opened on bottom, with plenty of fuel, I can see more air coming in than can go out the top which means it'll find the least resistant path to get out, ie. leak.
I don't think a seal would be a bad idea though, but I'd question that automotive tape providing much in the way of a seal, but maybe it would.