I've fabricated and fitted a front curved spoiler, engine under-tray, rear diff cover and rear diffuser of sorts (similar to that on the new roadster sport.

I was initially worried about over heating, however I think the net result has been positive... with the air flow bieng better managed around the engine without impacting engine running temperatures.

Are you going to mount the front up onto the front splitter you have or just a single plate onto the front of the chassis?

BTW.. the rear of the bonnet is where the Richard Thorne Roadster GT car took it's air intake from.. (2 x 70mm feeds into a reverse fed manifold using an Ali fab'd air box sealed to the bonnet where the heater box is at the moment) but it was designed to run with the windscreen in place... thus creating the high pressure zone.

You can see the bonnet cut-outs in the pic below:

[Linked Image]