Depends on how "real" you want your slings/straps. Flex-rex tends to be too thick and bulky for anything more real or scaled, but several people insist on using it anyway... along with thick metal rings to secure it to the weapon. It might technically work and be easy to put together, but imo, it looks a little like butt. Especially when you have a really good custom fig using it.
I haven't tried hockey tape, but knowing the material, it's a least better than the above. And it should have more flexibility, being thinner. Texture is probably fairly good too,
Yes... I use Lego "micro-canvas" (sails,etc.). Personally, it's about the best stuff for simulating scaled nylon or cotton webbing because of the texture. And since it's paper thin, it works nicely for
many scaled applications.
Buckles and sling attachment points are another animal all-together. For scale, try using hobby wire. 18ga is a decent size and very easy to work with. Styrene can be used also, but requires quite a bit more modeling skills to create proper designs/sizes. Not to say that
my methods/techniques are the best thing out there, but here are some examples for reference.
Every clip/buckle on the set-up below is made from styrene, with exception to the front "eye-bolt" and the rear "carabiner".
This one uses wire for the attachment and D-ring, with 1 styrene buckle -
Here's an older one, using wire and styrene -
AND of course this one -
Hope all this gives you some insight, and some good ideas.