Paul,
 I believe this is a good idea, but, (Yea but), you will have two problems. First: straightness, (and flatness) will be hard to get from sheet metal. The second problem bears from the first which is getting a decent seal, or small air leak will be tough. I am convinced that we DO need a fairly airtight throttle body to get a reliable idle. That doesn't mean that you couldn't do this with the hat sections made from aluminum or possibly an off-the shelf extrusion. The last question was did you intend the bearings to run against the sides of on of the pieces for side load?
Bill Jepson

Yes on the ball bearings. When you jam the throttle closed you have 15 psi 
or so in the engine on the side of the slide. Two, 2 inch holes, 6.28 square
inches, 15 psi, 95 pounds of clamping force. That usually causes it to jam.

I am not worried about the leaks. When the throttle is completely closed
I think the idle will be well under 1700 RPM. You can always put a bit 
of  sticky back teflon tape in there to close up the clearances. 

Since this thing is only about 12 inches long the flatness should be adequate 
if done on a press brake as opposed to a box or pan brake.

Paul Lamar
 
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