It's a long time since I raced RC-cars, and I never did off road stuff, but I think trying to develop a airfoil profile for these sort of application has little benefit.
I just looked at some off the photos, you posted and from where I sit I don't think you would gain much by developing a "wing" for this application - not in terms of overall downforce, compared to the spoilers/airdams you run now.
In a nutshell, you just try and "shovel" as much air on top of the spoiler as you can and try to keep it there with the side fences/endplates to get some stagnation pressure.
Nothing wrong with that, it's a bit "draggy" but that's about it.
If you would go to the trouble and develop a "real wing" (airfoil profile) for a buggy, you will have problems, to keep it at the optimum angle of attack most of the time, because the car jumps and bounces/pitches around quite a bit.
I doubt, you would see large improvements in overall downforce, and just to gain some points in efficiency (L/D) seems to be a lot of cost/trouble for very little gain.
I don't think, you will gain much from mounting the spoilers directly to the un-sprung parts/uprights etc.
you will get the same downforce either way, and I don't think, you have problems with bottoming out/ or worry too much about rake changes (change in the pitch attitude of the car).
The reason F1 or real race cars (on circuits, not off road racing) are concerned about this is, because they generate most of their downforce with the underbody/diffusers an/or front wings, which both operate in ground effect, which are extremely sensitive to changes in ride height/ground clearance and angle/pitch.
If you want more downforce, make the spoilers larger, if this is permitted by the rules (I don't know).
Alternatively, if you are "crazy" enough, and the rules permit it, you could try what used to be, maybe still is, I don't know
a solution found in slot car racing - open class
look at the car in the middle&right
they used to run longitudinal air dams, to try and increase the pressure on top of the whole car surface.
at speed this air dams would bend down sideways a bit, increasing the absolute planform area.
as a side note, as you see, they did run skirts a well to seal the floor to the ground and run between 1-3 mm ride height (dependent on tyre wear).
These things were seriously quick at the time > 100km/h
Have fun & good luck
