My Kindle Fire is a typically featureless black slab with one button, two small speakers, and no fasteners. After a few days in my pocket, the upper-left corner began collecting dust on the inside face of the cover glass:
That’s not terrible, but it does look ugly and lowers the contrast a bit in that corner. As nearly as I can tell, the speaker grilles provide the only way for that dust to get in, although this was a refurb unit and perhaps the seal around the rim is broken.
In any event, the speaker grilles look like this:
I slapped a strip of 3M Micropore tape over the openings as a stop-gap fix:
After a few days, the dust wasn’t getting any worse, so I ran a scalpel blade around the speaker opening and sank the tape atop the grille:
The advantage of Micropore tape is that it won’t completely block the already feeble sound from the speakers.