The latest FC rules have a well written section controlling the flatness of the undertray, and it also describes the measurement method. I can't see any way around it and the intent is captured completely in the written rule. You might consider adopting the exact language. I also think you want the undertray to be
flat but not necessarily
horizontal, as chassis rake can be a powerful tuning tool.
If you want the entire outer sides of the sidepods to be flat within the 1" tolerance to the outside of the wheels, then you need to say that clearly, as what is currently written is ambiguous (it only says continuous, not vertical or flat). How about the top of the sidepods? Can they have curvature and deviate from a roughly horizontal surface?
These details, like the "sportscar nose," are where I can see lots of potential for aerodynamic development and advantage. You probably want to clearly define the limitations but not necessarily stifle innovation.
I really think you need to list the allowed dampers, and limit it to those that cost less than a certain amount. Let people change the internal valving as much as they like, that's a good place to let owners experiment.
I have mixed feelings about an independent rear suspension. It will make the car more expensive, but it, with a differential, is pretty much necessary for an entry level "formula car." There are ways to make a solid axle work well, but it's still a hybrid of a kart and a formula car, and will reinforce driving habits that will have to unlearned for those moving on to faster formula cars.
I like what you're trying to do, and I understand you have a difficult task making existing designs fit within the new rules. Please excuse my messing up your sandbox! At some point I'd like to design an entry level formula car that costs under $20k ready to go on track, and maybe F600 is the class that allows that. I know it's possible, but I've got a very full plate for the next couple of years at least, so I'm reading these proposed rules with only academic interest. That means I'll share any potential loopholes I see.
Nathan