Microsoft does not officially sell rubber strips separately for Surface Books.
Even through Microsoft service channels, they typically replace the whole base — not small cosmetic parts.
So your eBay strip is fine if:
It matches exact length
Thickness is same as original
It sits flush when placed
Since it happened exactly after the strip came off, this is almost certainly harmless vibration — not a failing speaker.
You may also try thin 3M double-sided adhesive tape. Never use Super glue, Fevikwik or, Strong epoxy