Taking four filaments (2 pairs of two) and putting a sleeve around wasn't as hard as expected. First, using two cable straps, I separated the filaments I wanted to use. They were easy to push underneath the fabric, and have rounded ends:

Then I fiddled them into a sleeve and gently squeezed it:

The resistance is a bit too high (I expected about 5 Ohm), but not that bad:

It turned out that I should have shortened the aramid filaments before adding the sleeves, like for the next sleeve in this picture:

I thought about putting more or less than fours filaments into one sleeve, but with two it might get too crowded, and more than 4 is probably just too hard to do. Also, four will be fine for either brazing or clamping, so I'll stick with that.