What would you do by researching on the internet, buy the rain gear that goes on DH bikes? There is not a whole lot of options for bad weather gear for the kind of bike you have. And that was probably the closest they had that would suit your bike. In the rain you're going to get soaked no mater what you do to prevent it. While the fenders will protect you some from the spit up from the tires, it won't do a thing for the falling rain. You might want to try making your own. What's not practical about a rain coat?
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Appreciate the old bikes more than the new.