IME they need something less than 1/2" diameter to cling to since the leaf stems (petioles) wrap around the supports to allow the clematis to climb. Plastic netting is difficult to remove the dead vines from IME, so I tend to use narrow metal supports like 3/8" rebar, metal cable, or wire mesh fencing with at least 2" spacing. Compostable string would work as well since you could just cut the vines and remove them with the strings. Fishing line needs knots to keep the vines from sliding down.