If its not in the center your heat will not have even distribution. The air will get to it in the center (my basket is only 14" in diam), so don't worry about it. I have 6 - 3/4 " holes around my base, and it wasn't until that I drilled #5 and #6 that my heat became easier to control, using good ol fridge magnets. You need all that air in some weather, and it's easier to close up a hole if you don't need it, than create more air flow when you REALLY need it.
Holy crap that a lot of holes!!
Even at 20 degrees four 1 in. holes are enough for mine and if I open them all my UDS will run away from me.
I tried the magnet thing did not like it. Four 3x3/4 in. pipes, one valve and 3 caps seems to work best for me.
...but yes basket in the center, for sure.