Refreshing the rear of the car this winter, so in the process of stripping down, painting etc.

One thing I am thinking about is making a bottom 'diffuser' like cover to stop the parachute affect.

When at full droop my wishbones are lower than the floor line so I am going to either notch a cutout in the floor, or make some lowered sections to cover the wishbones in droop.

Any thoughts which is the best option? is a hole better than a section protruding in terms of the airflow?

When I did mine it was really easy as it has a live rear axle. My locost is almost completely flat underneath. The diff' casing pokes through a 8 x 4 inch hole by about 1/2 inch. Axle movement is very small and it's handy for a trolley jack to be used. There are 4 tapered aluminium fins underneath, bonded and riveted in place. The difference it makes is huge and was well worth the effort.

With independent suspension you will have more gaps to allow for movement, so I think I would make a card template first.
The rear section (under the axle) I made out of 1.2mm galvanised sheet, I just happened to have, was intended to be a template. When I made it to the correct shape/size, it actually didn't weigh much so it went on! It has been on a year or so now and working well.