They do look cool! I am building one because I am removing the rear bumper and I need to add something back in. The lateral face of a diffuser should be as long as possible going as far forward (think rear end housing) as possible and even protruding past the rear body lines of the car (that is determined by wind tunnel testing to determine the flow patterns and the best location to rejoin the two flow streams). The lateral face should also be at an angle up towards the aft end of the car, 10 degrees is a good starting point. The vertical plates should run parralel to the length of the car, assuming the lateral face has enough up angle. If the lateral face is horizontal then the vertical faces need to be wider at the aft end of the diffuser than at the forward end of the diffuser. The vertical plates are there to try and straighten the air as much as possible.

A diffuser is the opposite of a nozzle, think of your sprayer on your hose. To get the jet it goes through a nozzle. A diffuser wants to slow down the air.

Thanks guys for all of your imput. The main reason I wanna do one is just for look. Now if its functional then thats a total bonus. I just wanna make sure that im not doing something that is gonna negatively affect my car. Plus i was interested in how some of you may have made yours.

Bryce im a huge fan of your build so thanks for the input. I love small body fords. I think that removing that clunky rear bumper will be awesome for your car. Yours will definitely be one of a kind. Thanks again