Here are a few ideas:

Possibly the biggest platform and strong, but not the prettiest ...

http://metalmule.com/by-brand/yamaha/yamaha-xt660r-xt660x-luggage-rack.html

Strong and looks a little more "stylish". Cheap with current exchange rate...

http://www.off-the-road.de/en/XT-660R-X/Luggage/OTR-rear-rack-Yamaha-XT-660-R-X.html

Or the genuine option...

http://www.yamaha-motor.eu/uk/products/motorcycles/adventure/xt660x.aspx?view=accessories&partcode=5VK-W0736-10-00&year=2014

Alu ones aren't as strong, but are obviously lighter...

http://shop.touratech.co.uk/luggage-rack-aluminium-yamaha-xt660r-2006.html

Pricier version...

http://shop.touratech.co.uk/luggage-rack-aluminium-yamaha-xt660r-strong.html

I'm sure there are others out there, but the above should give you some food for thought.

__________________