I used a Jeg's universal oil separator on my car (didn't have $100 to drop on one of the ready made ones) and plumbed it with 3/8" fuel hose.
It works great, but the problem I'm having is that after the car sits for a little while the hoses have a little droplets of oil form along the length of them on all sides of the hose. If you wipe it off with a paper towel it just comes back after the next time you run the car and it sits and cools off. I've never noticed it while the car was running, only after cooling down.
I figured fuel hose would work great for this but apparently not. Do I need a different type of hose? Do the aftermarket separators/catch cans all use that hard plastic hose?
You are probably using some type of heater hose and the oil is going right through it. I had the same problem when I installed a UPR can on my 14 GT. I didn't use the supplied hose because it was half collapsed so I went to the parts store and bought some heater hose not realizing it was not suitable for oil.
I later had to go back and get some PCV/emissions type hose and didn't have anymore problems.