Especially with an air-cooled engine, it's tricky to make a diffuser without some big compromises.
But then, diffusers on production cars -- or any car with a relatively soft suspension, which includes just about all cars modified for racing -- are going to be compromised by the changes in ride height from side to side when you want them to work, which is in corners.
I think most diffusers on contemporary production cars are there to make the cars more stable at high speeds in a straight line. This makes sense, since flooring it on an open stretch of pavement to see what the top speed is like is going to take place on a lot of these cars, whereas really testing the limits in corners will happen on relatively few new Porsches. 99% of Singer owners are going to break the limit on a straight stretch of freeway, for example, while maybe 1% will do any meaningful racing or time trialing with their cars.
So the demands of a splitter on modern production cars is going to be worlds apart from what is/was done with ground effects race cars. Those cars (as I understand it) needed extraordinary stiff suspensions to keep the whole of the car's underside at a specific height up off the driving surface, especially in fast corners.
I experimented with a rear diffuser on my air-cooled 911. It required openings for air to pass around the cylinders and heads, and I didn't have as much control over specific angles of the underside of the car as I would have liked. But I think primarily because of body roll (even with 600# springs) it made the car unpredictable in corners -- which is why it has a lot of dust on it in the picture.
More specifically:
More recently, I took a more modest approach. The changes in ride height are obviously greater the farther away you get from the center of the car. So I made a flat panel down the middle, with vertical pieces that bolt on at the track. These pictures were taken while I was making it. I've made two versions -- one drops down closer to the pavement than the other.
In fact, all my aero folds up and fits in the car for the drive home:
