Thanks for the advice so far everyone.
I may have had a breakthrough today. I was examining the stock fascia when I noticed a few evenly spaced holes (from manufacturer) that might be able to hold a screw or zip-tie. Pics below.
The only downside is that due to the location of the holes, any air dam mounted there would cover the outward-facing lip design that I believe is beneficial to airflow (downforce). Hopefully I could replicate that shape with a homemade air dam.
It seems fairly straightforward that small nut and bolt would do the job here, but if anyone has any suggestions for the best way to mount an air dam from these holes, I'd appreciate it. (Potential first mod, unless you count the tonneau not installed by me).
Thanks for reading.
Almost forgot: I've been rethinking the design, and am wondering if mounting it from behind the original fascia would be more discreet and still provide some of the same benefits. Got the idea from the design of a Toyota Corolla I saw.