If you’re involved in plastic design you may have heard of Protomold. If not, they’re a Rapid Injection Molding outfit. You can send them your 3D models and have a prototype created super fast for as little as $1795.
If you just completely glaze over when somebody mentions plastic design or want a interesting purple cube sitting on your desk, they have a free sample they’ll send you. It shows you good and bad practices and what to consider when creating a RIM part. Really cool.
You can go get a sample cube here.