Whelp, after realizing I had neither the time nor the expertise to do the job correctly, I bought some industrial-grade hook-and-loop fastener and took my little powered sub and Spark EV to Lombard's Stereo in Santa Maria and this is the result:
I figured since nobody can sit in the middle position anyway, that's a good place for the sub. No reduction in leg or foot room, practically invisible wires, and no holes drilled toward the traction battery! Those of you looking to mount an amp may want to consider this position as well.
I'm pretty happy with the result since I can now hear the bottom half of my tunes.
One thing to be aware of is that the head unit doesn't have a discrete "power on" signal: it uses the GMLAN bus for that. So if your amp does, you'll either need to tie it to the ignition signal, or use a low-voltage trigger which lets it stay on any time the speakers are energized. I opted for the latter so I can still have bass without having the car on.
If I understand this correctly, this is mounted with "Industrial Strength" Velcro?!?! Please realize this is a big no no, a lethal projectile in the event of a crash. No Velcro (that I am aware of) and wires are going to keep a heavy object from going whatever direction force sends it, whenever the vehicle it is "installed" in, goes from <15mph to 0mph in an instant. I'm still MECP Advanced certified by the C.E.A., even though I haven't installed professionally since 2015 (I did 8.5 years big box install full-time), I'm not talking out of my arse.
On a different note, I want affirm what has already been stated/alluded to, there is no way a 4" , 4"x6" , or any speaker with a single voice coil, can produce a "decent" full range of sound. I really liked the pizza analogy someone used, where essentially some people are fine with a frozen red baron (any cheap speakers with limited range, that don't distort at moderate volume will do), some demand fresh organic brick oven pizza (separate high, mid, low speakers to help achieve high fidelity), and everyone else falls somewhere in between.
When I finally acquire a Spark EV (fingers crossed, test drove one, love it), I envision starting off my speaker changes with an old pair of 4" roadmaster two-way speakers (yes, this is most always a garbage "Wal-Mart" brand) that I acquired in a trade, which are amazing for what they are, and a pair of Pioneer TS-X series enclosed 3 ways, probably secure mounted (no velcro) under the driver and front passenger seat, firing backwards. This should achieve a decent range of sound, with little distortion at high volume levels.
Please understand (if you don't already) the law of diminishing returns certainly applies to car audio. If you will settle for nothing less than high fidelity, with high volume, professionally installed, you're well over four figure territory (in this case, you're probably not driving a Spark EV in the first place). On the opposite end of the spectrum, you probably want to stay clear of the flea market/no name stuff, unless you are 100% certain you know what you are getting. Anyone who is willing to give up a chunk of the little bit of rear cargo room that exists, for a decent slim 8 or 10" sub, likely won't be disappointed in the difference I know it will make in this car.
I've learned so much from this forum, and am grateful for everyone who has constructed and contributes to it, Thank you!