nothing? 
give the one i linked above a try 
way less bulky than the original, 19 g vs 51 g (not as sturdy of course, but looks still ok to me) and the silicone rings are less tight
O M G!!!
I just received my Astrolux S1… its short tube matches perfectly the threads of the Skilhunt!
I had never heard about that! I can’t believe no one have tried this before :s as you can see there’s the same problem of the orings that are way out of place, but it’s still an awesome surprise for me! 
…and here is this weird frankenstein 
have a look here: Short 18350 tube for Skilhunt H02 and H02R: are you interested?