Hello! It is my first post so I would like to say hi to all the members. I am glad this forum exists 
And straight to my problem. I have a quite slippery and soft nylon twine, this one:
Braided Nylon Seine Twine #24 - SGT KNOTS - 100% Nylon Fiber (https://www.amazon.com/Braided-Nylon-Seine-Twine-Construction/dp/B01FGHTEEE)
It's very strong and a great utility twine. I use it all over the place but the reason I actually bought it was to tie together some snap bolts to of my diving equipment. Most popular way to do so is: https://www.tdisdi.com/tdi-diver-news/how-to-attach-a-bolt-snap-to-dive-equipment/
However, I need to tie a snap bolt to something much heavier than a flashlight and hence #24 (1,85mm) nylon twine. But the problem is, it does not hold very well with the square knot. It comes undone. It is slippery and the knot has to work a little - the tension is not constant. Melting the ends works quite well but I don't like the thought that it is the only thing that holds it.
What could be an alternative? My only requirement is that I need to be able to pull it strong together before finishing it, the twine cannot be loose.
Thank you!
This is a tough problem, especially with knots underwater that makes the line more slippery. Constrictor-style knots may have trouble since rotation of the object may cause the crossing patterns to slip off a convex shape and loosen.
So instead, I would make plenty of wraps, then plenty of frapping turns at 90 degrees between the two objects to get things really tight (as seen in lashings). Then, you could twin the two line ends together and tie a simple overhand knot (
European death knot), and have a needle on hand to insert the point into the belly of the knot and roll the twinned overhand knot tight against the objects while you maintain some tension.
I would practice rolling an overhand knot with a needle or nail to get a feel for the maneuver first.
I tried it with some similar nylon mason line and it worked quite well.
