Often there may be many correct methods that work so I'm not implying mine is ideal or better than someone else's but it is what has worked for me and others i have suggested it to.
I have not had to use a crimping tool but what has worked is to twist one or even two small 1" or even 1.5" long bare wire scraps to the end of your wire. I've used the same 16ga wire and just cut a couple scraps from it. After twisting together, trim the ends flush, and that larger diameter will fill up the inside of the tube connector more precisely so that it doesn't have to be cramped to the point of mangling. It's easier for me to solder as there will be more wire contacting the inside wall and less gap getting possibly filled with solder. It also gives more surface contact against the tip of the male tube connector.
If I recall correctly the one piece of 16ga wire plus 2 more 16ga scrap pieces fills up the space fully, but even one scrap piece (2x 16ga total) let's it be crimped with much less effort. Just play with it and see what fits, but the idea is to fill up the space with more then the one wire. More surface area contact, less crimp required, more tip to tip contact with the connector.
