Finally did a little work, though it's more SPL side of things (but interesting, none the less).
I wanted to find out the proper port for maximum SPL at the dash, sealed up (doors and windows all closed). Also, wanted to find out maximum SPL across the largest band for demo'ing. All testing was done about as much as my setup could take (probably 130dB-ish). Anything past that, I'd need a TermLab or something and do single notes. Everything was done with REW and a Dayton calibrated mic.
For all this testing, I built an multiple length port sections with the maximum opening my baffle is cut out for (14.25" x 5.375"). I had 3x 2.5" long and 1x 6" long to play with, so I combo'd those in every way I could! This was no port plate (about 1.5" port length baffle), 4", 6.5", 8", 9", 10.5", 13", and 15.5". Turns out, the first port I built (8" total length) was the best fit, hitting a hard peak at the windshield at 45hz. Orange is the 8" in that graph.
From there, I built a plate and adapters of 1.5", 3", and 4.5" and slowly shrank my port (keeping the 5.375" width) and measured each result for sealed up, window down, and door open. Mic was strapped to the passenger headrest. Kept the 15.5" length for this test. Found that by shrinking it to 9" (probably tuned around 29.5hz), I found the best band of 30-40hz and still did decent below and above, so will build a permanent port plate for that tuning (similar to my SQL 22hzish port, but more open and less zig-zag shaped). I also modeled up in WinISD to see how it compares... (I know, cabin gain, etc. but hey! Would you look at that? Just look at that!)
Finally, I decided to test what happens if I throw my body on my hatch with my SQL port because it resonates pretty hard in the mid 20s. As you can see, a ~5dB gain at 25hz and the volume isn't even maxed out... wow.
Anywho, attached are pics and graphs!