I fitted one, an "eBay canbus multi-bulb special". I had the option of a 38mm or 40mm (festoon?) bulb. I flipped a coin and picked the 40mm version. It was a little too big, but the metal holder easily bends a little to fit.
I specifically chose a flat type, where all the little LED bulbs point in the same direction. An you can direct the light in the fitting, by twisting the bulb.
But the light still seems to spill out too much for my liking, and I'm likely changing back to the original (much duller) incandescent bulb. The thought of driving at night, and a passenger flipping on the light, I'd go temporarily blind, my night vision is terrible.
I'm not anti LEDs, I fitted the Philips "extreme 360" in the sidelights, and I've got one in the boot. But I think the interior courtesy light diffuser is a bit rubbish, and it's too simple to pick a MUCH brighter bulb.
The pack of 4 bulbs from eBay were only a few quid. Although I understand the quality of "eBay special bulbs" can be very mixed. But the bulb is much easier to swap over than the sidelights..