Geez thanks people, I am getting a little swell headed here with all the nice comments!
How was it done? Unfortunately I didn't document the process or take any progress photos, so I will try to explain what I did as best I can. It might seem a little lengthy tho', so if you feel your eyes starting to glaze over I wont be offended if you hit delete and move on.
I started by taking a quick mould off a section of the Vans supplied top cowl to get the basic shape. I cut and bonded this to flat pieces of wood that were clamped to the inside surfaces of the fitted baffles (I'm starting to form a pattern or replica here of the finished product). I then removed this semi-formed pattern from the baffles/engine assembly, braced it and cleaned it up sufficiently to take another mould (remember, I haven't started forming the inlet shapes yet). I formed the corner radii in this mould using nothing more than auto filler and a suitably sized socket from my tool collection.
More sanding and cleaning up, then I made a rough 'plenum' from this mould still without inlets. This was trimmed and fitted to the engine, ensuring there would be enough clearance under the upper cowl. Rigid urethane foam was then used to form the shape of the inlets. The trick here was to fit and size the blocks of foam so that when the cowls were in place, the inside edges of the cowl inlets were just touching the foam. The inside shapes of the inlets were then traced onto the foam with a sharpie. Final shaping of the foam could then be done using the traced shapes as a reference.
Fibreglass cloth was layed over the foam shapes followed by lots of filling and sanding to blend them into the plenum shape, resulting in the finished pattern. The final two-part mould was then taken from this pattern using tooling resin and chopped strand fibreglass mat.
The plenum itself was made using fibreglass twill cloth, core mat and fire retardant vinylester resin.
Those of you who are familiar with fibreglass work will know this is very much the 'Readers Digest' version. There is a lot more detail to the process than described here. I have the advantage of being general manager of a composite manufacturing company, (Composites International Ltd) mostly doing aviation related work, so have the advantage of resources and plenty of experienced help to call on for ideas if I need it.
Tahoe...? Lubbock...? I have to confess I had to google these places to find out where they were

. Yep, they look really nice,....and quite a contrast to the temperatures we are currently experiencing in the middle of one of our hottest summers on record here in NZ!
If anyone wants more info I will do my best to help.
Clive