the Shiningbeam clip could fit (2.2cm). the xeno is exactly 21.5mm diameter
the p1a has 23.0mm diameter so only 2 commercial diffuser tips exist afaik but no diffuser lens cap. The 21.5mm is the Fenix standard and you get tons of addons like diffusers and stuff.
yep, one could build a cheap diffuser with Scotch tape or other (adhesive) film. folks on cpf who are dead serious about the EDC (which has to be as perfect as possible) enjoy the Quark (22.0mm). you can lego the light as you wish (different heads flood XML vs throw XPG, tints, Fenix diffusers, tailstand, momentary on, 2 UI's, professional clip, etc.). They're on sale everywhere (goinggear) because of the Foursevens rebranding.
Fenix LD01 (on sale), good choice too. (diffuser lenses, diffuser tips, etc.)
Xeno E03 (without clip) might be okay as gift but i never use my copy. When i am serious about lighting needs (for a particular situation/day/application), i use the Quark -- it's the most practical of my four 1xAA-lights. Has a nice saturated greenish tint which looks vivid on green grass/outdoors. the p1a .. we all know *why* we bought it. ( the p1a is not regulated on High-mode (Eneloop, 14500) which is a shame. runtime is 60mins; not 90mins as advertised. current draw is ridiculously high (2.0A on fresh Eneloop), no wonder it's so bright! ). it feels much better built than a Quark.