I don't know if anyone else here has this problem but transitioning between 1/4" barb fitting for Soda kegs and 7/8" beer nut for the sanke coupler is a pain!
What you can do is take a 5/8" NPT-M to 1/4" NPT-F flared fitting and cut the gas flare off. Cut it off right above the threads, but don't hit them.
A dremel can do it, but an angle grinder and steady hands makes quick work of it.
Make sure there is an o-ring in your 5/8" wing-nut/hex nut and tighten. Put a 1/4" barb NPT-M into the other side and run your 3/16" or 1/4" line. Remove the flare when you need to use a sanke coupler.
I've been doing this since January and have had no leaks (including the gas) to date.
I can put some photos up later if anyone is really interested.
Now, back to getting the s$@*« spear o-ring compressed and getting the snap ring on.