SKS's frame bag is an undeniably cost effective way of adding some extra carrying capacity to your bike with very little extra weight or work for that matter.
Simply attach the two Velcro straps around your bike's top tube - ensuring you don't clamp down any cables at the same time - and then tension the bag by attaching the bottom strap to your down tube, again avoiding any cables.
The bag is designed to be at the front of the front triangle, making best use of the void between your knees, while also distributing a little of your carrying weight towards the front of the bike. It also leaves space on your top tube for front-mounted bag there as well.
The design is simple but works well, with a your stuff easily accessed via the waterproof side zip and can carry a surprisingly capacious 1.34 litres, although in a slightly awkward shape.
The upside to this shape is that unless you have a tiny frame, you'll still have space for two water bottles, and should be able o.
The reflective strip around the edge of each side of the bag is a nice touch and the bag itself only weighs 158g.