Rain cover for hiking backpack -- can I make/buy one?

I am starting a tour on Monday on this Brompton folding bike with the Brompton bag on the front and standard hiking backpack rigged to the rear with bungee cords. This setup works except that I don't have a rain cover for the backpack. I was going to use trash bags but that will get expensive and seems wasteful, and I can't justify spending $30 on a simple rain cover.

Any suggestions for waterproofing my bag?

[And since someone will ask ... I almost bought a used set of Ortlieb front panniers which would have fit on my rear rack, but the backpack works better because I can wear it when my bike is folded.]

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Permalink Reply by Dave Grossman on March 14, 2013 at 3:14pm

Pretty sure REI sells covers....give them a holler.

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Permalink Reply by Kevin Mulcky on March 14, 2013 at 3:19pm

You could possibly make a simple cover from a rain slicker/poncho from your local thrift store.

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Permalink Reply by Apie (11) on March 15, 2013 at 3:01am

Garbage bags are expensive? BROMPTONS are expensive! garbage bags are cheap. Try the dollar store for kitchen trash size, they look like would fit, or go all out and get 20 heavy duty GLAD bags for $5. How long is your tour for?

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Permalink Reply by Robert on March 15, 2013 at 3:07am

Try a spray on. It's called NIKWAX. My friends have used it and said it works great and very well priced. nikwax.com. My friends swear by it. Hope this helps.

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Permalink Reply by Thunder Snow on March 15, 2013 at 4:19am

http://goo.gl/roO53

http://goo.gl/PH1YP

http://goo.gl/KjMvu

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Permalink Reply by Anika on March 17, 2013 at 3:42pm

Fabric stores now sell all sorts of waterproof fabric for making cloth diapers with. A yard of that with a bit of elastic and you have yourself a bag cover.