This is what ive been using for the last couple of years......halfords 3 man tent.came in a kit of tent and 2 mummy sleeping bags for £100....light weight and packs small.fits in the roll bag in front of my top box.along with one of those foam roll up mats.had some heavy scottish downpours last year and no leaks...has a porch to put your biking gear, nice to have somewhere to put your wet gear....oh...I make my own pillow from my jacket thermo lining...did the job....a few beers help for a good nights sleep aswell 
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One, is never going to be enough.....