This will also work with a carto, I've tried it:
ATOMIZER MOD
If you have problems with carts that don't fit your 901 very snugly (they are probably 510 carts), just get something like these Kobalt 8" long-nosed pliers from Lowe's:
Put the very end, just a mm or so, of the cart end of your 901 atomizer into that circular void by the cutting blades. Squeeze very gently until you see the tiniest bit of flex in the metal. You're not trying to make a visible change to the atomizer, here, just flex the metal a bit.
Now give the atty a 1/4 turn or so, and do it again.
Test.
After doing this, my carts fit almost too tightly, but that's perfect for dripping. There should be no visible sign that you have done anything to the atty.