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But anyways, this is a nice thread and I have found interesting solution of it.

 

rubber plaster molding is a very good way. The alloys used for casting are closer to die cast alloys than the alloys you would use for a machined version. Rubber plaster molding has very good surface finish and tolerances, not quite as good as die cast, so you might need to do threaded holes or whatever as second ops.

I have used these guys before, and they did a good job

http://castingsnet.com/d/page.php?id=1512

 

Go for Rapid prototype is good and cost effective....

 

It all depends on the geometry of your part and what the requirements of your prototype are.

-If you just want something to hold and show off, and possible do some fit tests, just make an SLA or SLS printed part.
-You could machine it if it's possible with your geometry and have a nice metal part for some testing.
-There are lots of ways (some discussed above) to make cast or molded prototypes. It all depends if you plan on doing functional testing, etc.

Check out Protomold, they do quick turnaround, one-off parts for quite cheap for a variety of prototype processes.

 

Due to your part is very huge and thick, so any metal tools would be very expensive, sand casting is cheaper but it needs lots of polishing works, and very not strength, i had ever saw a similar case was make of extrusion part and welding...pls send me drawing for reference if you don't mind.

 

If you have the step files prepared, you can use a 3D printer to create a quick prototype of your part.
In regards to mold flow, tolerances, and other details... work with a design engineer once you are in that phase.
Let me know if you have questions.

 

It depends on what you need the prototype for really. Is it for functional testing or just visual?

As kinnavate alluded to - you might not get a great accuracy using casting.

 

You can send the 3D drawings here to clarify the strength and appearance requirements of the prototype, so as to give you accurate suggestions. Generally speaking, the fastest and cheapest way is to use 3D Printing or CNC Machining.