The Smoktech dct cartos will work. Just realize that any cartomizer is going to appear to fit tightly with the Phiniac endcaps, they were designed to fit cartomizers tightly.
Lubricate the cartomizer with some e-liquid before assembling, and use a twisting side-to-side motion instead of a straight-in approach.
I also HIGHLY recommend using a tank accessory tool such as the Slap Yo Moma tool or the Siam Mods tool sold by Phiniac to protect those important o-rings, especially because how tight the carto's fit.
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