Outdoor mirrors to fix on a wooden stable wall beside the arena

[JillA]

I have a handy wooden wall which although not huge is big enough to take some mirrors and adjoins my manege, as in this picture

It should be enough to check on seat and horse's way of going, and free standing ones are out of the question for a variety of reasons. Any idea of what and where to buy something that can be easily fixed to it and survive outdoors please?

 

[CBFan]

Buy a stable mirror or two side by side? I picked up a big one that I think was 3ft x 4ft or something...