Damage enchanted Curly Mask felt fair and balanced.
Didn't bother filling out my board, just sat on 10 gold so I could pay any Stinger's Lab that showed up (btw Portal being buffed by Lab is absurdly OP).
Assuming the other problem poison items gets brought into line. That should make it work. But honestly, it's not uncommon for me to go portal + only one poison item. The low roll (but still very potent) poison build I tend to end up with is Shout+Mask+4s charge 1s item on the same row. Then just a portal and random trigger slow/slow on start/stun to slow down the enemy items. By the time your non-rushed poison items would have triggered your poison portaled mask will have already killed anyways. That build wouldn't be impacted by that change at all, same with any good poison blade build. The other poison items just don't matter in that.
I'll make the poison portal harder to find, it will spawn at epic level. This way you at least have to gamble with your life or make a transition. I'll also change the poisonous-blade, no more scaling on slow. it will scale on poison activation, that way you have an incentive to stack poison items rather than a 2 item combo.