Just checked. They say 65 mm diameter but Q8 is a tight fit. Took me like a minute on the first (slightly drunk) try. Surely could be done way faster with experience. L would be much easier but then it would be so large…
I’m not sure which one to recommend but probably S. 
Sounds a bit like my prom night.
I bought a cheap lambency diffuser for mine. It works, but does create the bare light bulb effect so I haven’t used it much. Indoors,ceiling bounce is usually preferable to me. Otherwise I’m patiently waiting for the BLF Lantern.
Wellp, anyone wantsa get crazy, get a clear jug of water, set it atop the Q8, and add just a few drops of milk for the amount of “haze” you want in it.
The bigger the jug, the more diffuse the light, and less bare-lightbulb effect.
Me, I’d just stick with ceiling-bounce.
I made this one from a plastic tumbler. I just sanded it to dull it up and diffuse more of the light. Took only 10 minutes and didn’t cost a dime. Already had the cup sitting in the cabinet unused. I am not a fan of most diffusers. Too small and too milky white looking for my taste. I like warmer looking for out on the patio table or deck.
disposable plastic cup
Can confirm…fits and works well.
I saw a thread about using an orange Gillette Fusion shaving gel cap to warm up the light from a Streamlight Siege lantern. It got me wondering. I had a can of the stuff under the sink, so I took the cap and set it onto my Q8. Lo and behold, the cap sits just inside the bezel (it fills the space, but it doesn’t readily “snap in” or hold, so it might only work for stationary tail-standing) and this cap creates a wonderful lantern light. What amazed me the most was the lack of glare! Apparently the curved, dome-shaped cap does not send as much light straight sideways as one gets from a vertical diffuser surface. I have a white diffuser tip that came with a Maratac AAA twisty, and that thing makes way more glare than this shaving gel cap on a Q8 ramped up about 2/3 of max. Not sure I’d want to use this cap on max 5K lumens, it might get too hot.
Until the LT1 ships, I think this might serve as a passable substitute while camping inside my cargo trailer. I have a high shelf built on one side that is just above eye height. For outdoors too much light will be wasted to the sky from this rig, but I have a Siege on the way also.