Someone asked me which LEDs are best for switches--twice now, and, well, I prefer rectangular 2x3x4mm varieties for three reasons:
1. they snap into the switch cover and sit there tightly, requiring no modification. Most don't even require tape when flipping over the PCB.
2. they have good light dispersion and high viewing angles
3. again, no filing required, and they are the most compact: should fit under most keycaps without interference.
But, photographs are more helpful, so here are a bunch for your reference purposes:
3mm flat top, 2x3x4mm rectangular, 3mm (t-1) round flange-less.
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Flat top (after filing the flange a bit) -
(I've found that these interfere with low Cherry profile keycaps in the middle rows)
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3mm round (flangeless) -
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2x3x4mm rectangular -