I ended up shoving a ton of teflon washers inside mine to get a good enough seal. 1/2 compression was too small, 5/8 was slightly too large. I used 5/8 and packed 2-3x 1/2 teflon o-rings inside the compression nut and it seems to do a fine job.
I used bargainfittings... just checked my order history and found these:
2x = 5/8" Tube x 1/2" Female NPT compression adapter stainless
6x = O-RING 1/2 NPT - SILICONE
If you aren't using any kind of quick disconnect and hose clamp fitting works then i'd go that route. It's def cheaper. Each compression fitting was $26 and I question if it was worth it over ordering a copper one or another brand CFC with NPT threads already.