HI we are about to invest in some arena mirrors and would like some advice on the best positions in the arena and the suppliers we are in West Yorkshire.
upright at the end of the long sides (for checking lateral work) and sideways along a short side, or along behind B or E, are ideal imho.
can't recommend suppliers, but i would say, don't go for plastic ones, they distort terribly and from a distance the image is utterly rubbish.
they're worth spending money on to get right, imho. make sure whoever puts them in realises that the uprights they're fixed to have to be perfectly parallel to the edge of the school too, i let some muppet builder put them in at my old place and although it was only angled a tiny bit, it made it utterly useless when i was riding down the long side... couldn't see myself at all until i was within 15' of the mirror!
The link to the mirrors for training guide has some ideas about erecting them outdoors - basically attach them to some plywood and stick them on some posts in the ground/attached to the fence.