I'm just getting into bike packing and I've made a frame bag and I'm using an OR lateral dry bag as my handle bar bag, but I haven't pulled the trigger on a seat bag yet. I have a little trouble justifying spending $150+ on a bag and I don't think I can make my own, though maybe eventually... I'm thinking for now of using a seat post mounted rack with a waterproof stuff sack strapped on it instead. Anybody have an opinion or experience with this? I'm thinking of this rack
http://www.rei.com/product/831420/delta-cycles-usa-post-haste-seatpost-bike-rack-special-buy I already have an OR waterproof stuff sack and the rack is pretty cheap, my main worry is the rack breaking from the repeated stress of bouncing down the trail but I don't know how realistic of a worry this is.
Thanks for the advice