I wanted real glass mirrors rather than the acrylic type, because I've seen acrylics with such bad distortion that you don't even know what you're looking at. I got a quote from an equestrian mirror specialty vendor online that was, no joke, $30,000 for a 66' wall of 4' high mirrors!!!! And that didn't even include the frame. Obviously that was out.
I also considered using mirrored closet doors and building frames for them to slide into, after reading that idea online. It was intriguing but I was worried about how the MDF etc around the mirrors would hold up outside, because it's really not meant to get wet. Also, unless you luck into them on Craigslist or something, they aren't cheap, and I wanted a whole set of matching ones.
What I finally did was contact a local general glass and mirror company that was well-reviewed on Angie's List. The price they gave me was only twice that of the closet mirror idea, which was great. They had done one or two previous arena mirror projects before so they weren't specialists but had some idea what they were doing.
I'm not handy enough to build the massive frames myself, so I asked the Amish guy who installed all my fences if he'd be up for it. He is reasonably priced and super easy to work with, and it is basically just carpentry work plus post driving, which fence installers are really good at already!
The mirror company provided the fence builder with a working design, and when it was in place they came to install the mirrors.
After doing some research I chose to leave the mirrors free-floating in the mounts rather than gluing them to the plywood. I was concerned about the plywood swelling and bending with changing moisture levels, and causing distortion in the mirrors if they were glued to it. If you don't glue the mirrors do make sure you get ones with safety backing.
Substantial post-installation tweaking was required to make the mirrors fully functional, and even now they're not perfect. The trouble with using wood posts is that they are not 100% straight and will bend and flex over time. It's also difficult to drive posts into the ground with minute precision. Since the mirrors are so large, small amounts of deviation in the angles can make a big difference.
- Vertical angles: The mirrors are set 4' off ground level. I read and was told that at that height they would not need to be angled, but upon installation I saw that they did need to be tilted back/up to be visible from farther away. The fence builder routed out the front edge of the frames to allow the bottom edge of the mirrors to slide forward.
- Horizontal angles: Due to some variation in the posts and frames, the mirrors didn't end up exactly perpendicular to the long side of the arena. On one side you could see far beyond the edge of the arena and on the other side you could be on the track and not see yourself. The fence builder adjusted the frames again and also put sealant in one corner to hold the mirror at a different angle. It's not 100% perfect but much better now.
The mirrors have been up for over 3 months now and look just as good as the day they were installed. Hopefully that continues. Some people will tell you you can't put mirrors outside, but my trainer has had them for 20+ years and only one has had to be replaced.
Update 4/26/15 - Here are a few pretty spring pictures of the end result: