Actually, when your car system is heated up and pressured it will push it out into the overflow up to the HOT line, when it cools off the pressure will reverse and the coolant will sink down to the Cool line. So if you have been driving around a while and pop your hood and your coolant is around your Cool line, that means your low on coolant and you need to add until you reach the Hot line. But if your car is ice cold and your catch can has it filled to the Hot line then you're overfilled and it will come out the open curve tip at the top when it gets hot
My Black Magic fan came with a support bracket for the stock coolant catch can, I just ordered a new stock one cause the old one was cracked and splitting at the top.
That's what I was getting at. If the fluid level changes throughout the heat cycle, how can it be a good idea to delete the overflow.
I think I'll start with one of the $15 plastic ones from Summit and go from there.