As far as roll bar padding in 1/24th scale, I have had good results using 1/8th-inch shrink wrap tubing - I slit it lengthwise if needed, and a touch of super glue usually holds it in place well.
For the zip ties on the paddding and for underhood use, regular old sewing thread works quite well for me. I use a good set of tweezers to tie the knots, and a pair of small "decal" scissors to cut the excess closely off.