I dislike solo cups, they are even worse than trade pots for transplanting, the dirt and rootball sticks to the sides. People laugh at my cottage cheese containers but they are perfect starter pots as it's very easy to get the young plant out of for transplant. I'm only using the 3.5" pots as the others are too big when transplanting into 1 gallon pots.
I've seen people slit the solocup vertically then tape it. You can then just remove the tape and peel the cup from the rootball. I'd slit and tape on both sides.