Great question — answering as the UphillTech team:
Zipties (aka cable ties). Be sure to use ones that are UV-resistant. Cost pennies each, strong enough for the application you're talking about, non-damaging and non-permanent, and easy to remove when done.
Full answer by gregmac: https://diy.stackexchange.com/a/75323 (CC BY-SA 3.0).
Need it done for your business? UphillTech.
