So I'm going on a 3 day weekender and want the option of taking my half helmet and modular. The only option I have is bunjee cording something to the passenger seat. I have been looking at waterproof backpacks with helmet holders but the helmet holders are not fully enclosed. I would hate to have to put on a wet helmet for a long ride. If I need to just go for a helmet bag (waterproof of course) and put my clothes in a thick plastic bag in my saddles bags with my MSR fuel canisters I can. Anyone got any advice or products I can trust for under $100?