If the jacket isn't very thick or bulky, keep a set of shoelaces in one of it's pockets. Roll up the jacket and use the shoelaces to tie it to the back of your saddle. Kinda like a blanket roll on a horse saddle.
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HCFR Cycling Team
Ride Safe ... Ride Hard ... Ride Daily
2017 Colnago C-RS
2012 Colnago Ace
2010 Giant Cypress