Denis (with one 'n' now, sorry about that)
As Browney says, things like nuts & rivets won't matter so much, within reason, but it's normally big stuff like chassis members or suspension gubbins which scupper the effectiveness of a diffuser but i would also add poorly planned cooling system outflows and the disturbances from the wheels.
Ideally the cooling airflow should be completely ducted to the radiator/intercooler etc and then compeltely ducted out of the car, although some provision may have to be made for direct cooling of engine/gearbox or specifically sensitive components. Regardless, this cooling flow must not be allowed to leave via the underside of the car if the diffuser is to work well.
For the wheels it would be nice to have endplates which extend forwards to the front of the rear wheelarches in order to try and stop the peak suction at the apex of the diffuser drawing in air through and around the rear wheels. This will make an unholy mess, though will be handy for brake cooling.
Also important is to have a smooth-ish transition from the floor to the diffuser. A lot of people, including specialist car companies who should know better, manufacture it out of two flat pieces, with a join exactly at the apex of the diffuser. Unless this panel match is perfect you will get a gap, or even worse a step, in the floor which can knacker the diffuser. Best to have it made from a single flattened v-shape piece with the seam 100mm or more upstream, so that the disturbance from any mismatch has some time to sort itself out before it reaches the critical bit at the start of the diffuser.
browney
Yes a rounded front end will increase front lift but it doesn't have to be much of a radius at all since air is more forgiving when it first arrives at the car and hasn't had any of its energy stripped from it through viscosity and turbulence. Also you benefit becaus this further reduces the energy losses through the underbody and so increase massflow etc and you do get some of that benefit at the front of the floor. The 360 Challenge Stradale is quite a good example of this but you can spot it on the newer breed of Le mans cars too, which all tend to have a raised and sometimes radiussed centre section to the front splitter.
For more information you could try:
Competition Car Aerodynamics
Simon McBeath
Haynes Publishing 2006
ISBN 1 84425 230 2
or
Race Car Aerodynamics: Designing for Speed
Joseph Katz
Robert Bentley Publishers, 1996
ISBN: 0 8376 0142 8
Simon's book is more modern and has lots of cool pictures (done by our competitors at Advantage CFD, grrrr!) but has a misleading and technically incorrect explanation of how wings work, although unfortunately this is the version which is often written in textbooks. We're actually working with Simon on his next set of articles for Racecar Engineering magazine so we're putting him straight. Apart from that though his book is excellent. Both books are available from places like Amazon or Waterstones etc.
Cheers
Rob