Yeah alumide is just nylon with aluminum mixed into it. Alumide and nylon are both aquarium safe. I'm reading Shapeway's material list now an I think I would try just plain nylon first (it's even cheaper than alumide). They call it strong and flexible plastic. Yeah, you have to make a model in a CAD program. I mean, I think I can just set the model to public then you can buy it from Shapeways. They'll just print it out in 3D and ship it to you. If you're going to do that though, let me know because I need to incorporate some design changes from what I learned on this first part.
By the way, nylon and alumide is not CO2 resistant but that doesn't really matter unless you're designing a CO2 diffuser though.