I use Slick 6, it’s pretty thick.
Here are the strings I have made so far.
From top to bottom:
The first two are 6 laps. They’re way too thick for me.
The third is the type 6 cotton string I found on eBay.
The fourth is 4 laps. These are the ones that are closest (judging from the feel) to the store bought kind.
The last is 2 and a half laps. These are way too thin – even double binds don’t seem to work.