Ritchey Comp Bar Ends - 125 mm

I recommend this product

I've got these mounted inside the main grips on some wide flat bars, giving me a hoods-like position for long stretches. I've put some foam grips and bar tape on them, and they're very comfortable. I tried a few similar bar-ends for this purpose but these are my favourite. The curved end provides a couple of different hand positions.

One of mine is slightly tighter than the other so was awkward to fit, and scratched the bar slightly. (It's under the grip so you can't see it.) It's also surprisingly awkward to find a bar bag that suits this setup. However, for me, the benefits outweigh these issues.