I noticed in the update it listed a 3rd type of custom decoration being introduced and was worded something like "chance of getting when placing top 3 in derby."
This makes me think this is the only way to get custom decorations. I'm still waiting to get a 2nd custom decoration like a lot of us. Most people say they got their 2nd ones randomly from derby. I don't really like this, custom decorations look really cool and they seem near impossible to get.