I used a male mold made of foam.  Use the good stuff, not the closed cell styrene or the canned foam which is hard to work with.   After shaped, I 'paint' it with drywall compound sand it smooth then cover with vinyl tape or coat with molten paraffin as mold release.  

The rad duct was shaped based on my imagined airflow within the duct.  It is compound curves everywhere, not a simple wedge.   Much thinner at the edges than in center for example.  Is it better than a wedge?  Don't know, but the leaf blower flow test method showed pretty good distribution with higher flow near the back (should have been even thinner there).  Fixed that with a bit of ridge vent material  (porous coarse fiber stuff) stuffed in the back section.  Not the best solution there but I've not had water cooling problems so good enough.

Tracy

Sent from my iPad

On Dec 14, 2012, at 7:33 PM, Stephen Izett <steveizett@me.com> wrote:

> Hi Guys
>
> I am about to reinstall the Renesis engine in the Glasair SII and create the ducts to the exchangers.
> I plan to copy as closely as possible Tracy's 20B setup.
> Questions:
>
> 1. What is a good process? Shaping foam, sounds good. So how do you then seal it before applying glass/epoxy?
>
> 2. Tracy, why is your diffuser not a simple wedge? Do you believe you are getting a more even pressure to the top and bottom? Have you ever measured your pressure distribution?
>
> Thanks
>
> Steve Izett
>
>
>
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