[Jizvonius]

Custom extrusion is an expensive affair that usually doesn't pay off unless you need a lot of the extruded material (like hundreds of feet for a machine that will turn a profit). The machine time as well as the cost to create the die will get very expensive even for simple shapes. If this is just a summer project, I would suggest finding another means to create the parts you need, or modifying the design to use existing extrusions.

Either way if you still want to have the extrusion done, you would want to specify the alloy, the geometry(profile) and tolerances at the very least. This is all assuming that you know of someone who will extrude this for you.