Just stop.
While the drivers may face themselves, the resonance inside a subwoofer is easily nullified with some simple bracing, and if needed, fill. The Sonos sub is still a plastic fantastic toy compared to brands like SVS, HSU, Rythmic, Funk, etc etc etc. Also, it's way over his/hers budget.
OP, if you want to add components, you need to have components. Save your money for a proper receiver/amp, and 2 main speakers with some oomph of their own. Add a subwoofer later. I almost bought a HTIB system, but my brother talked me out of it, and now my system runs circles around his.
Child's toy.
Real Subwoofer
SheepStar