Not the answer you’re looking for but if it’s raining that hard it could just go under a emergency rain jacket (e.g. the Houdini ) you should have in the pack, or just spray it with some scotch-guard occasionally.
If I'm thinking it's going to rain significantly on a ride I'll carry a ziplock bag for my phone. You could do the same for the contents of your hip bag. Either but it it one large ziplock or a few smaller ones to sort the items. Freezer bags are pretty durable.
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