I used to concern myself with "squaring" the focuser too, until I read that article that Mike posted (as well as several others). I no longer worry about it.
My focuser is "fits all dia. tubes" setup. It's flat, with raised points at 2 edges for contact with the tube. I always cover the OTA when not in use anyway, The amount of dust that's going to get in through this gap between the focuser & tube as opposed to the open ends of the OTA (whether is use in the field, or covered when not in use) is not even a factor worth considering. (In my opinion anyway)
