This is my model of the Rocket League crate key (October 2017).

Since there currently exists only ONE image of the official key, no one has seen the back of it… I had to guess a little bit, but it's probably not far off from what the artist had in mind.

Print in two pieces & glue together, then paint for best results. See "paper.svg" and "paper.png". Paste into Word / Pages, then set the size to 33.25mm x 42mm. Just be sure your scaling settings are 1:1.