Wildcat Tiger Wayfarer: http://www.wildcatgear.co.uk/shop/seat-systems/wildcat-tiger-wayfarer/ and one of their tapered drybags: http://www.wildcatgear.co.uk/shop/dry-bags/tapered-dry-bags/ A bit spendy but very well made and secure.
I've ridden with so many people with wildcat seat packs, they always come loose and wiggle about all over the place.
Never had a problem with mine (Tiger Wayfarer + small tapered drybag) as long as drybag is packed tight and the straps are done up securely.
@turnpedal started
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Wildcat Tiger Wayfarer: http://www.wildcatgear.co.uk/shop/seat-systems/wildcat-tiger-wayfarer/ and one of their tapered drybags: http://www.wildcatgear.co.uk/shop/dry-bags/tapered-dry-bags/
A bit spendy but very well made and secure.