I recently rode in a Clearspan that belongs to my neighbors. It became apparent within 5 minutes that the metal supports on the sides were a giant hazard as they taper to the bottom so my horse happily fit under them but nearly decapitated me. My trainer ended up putting barrels, chairs, muck buckets and whatever else she could find in front of each support to keep my horse away from them.
Now granted, I was on a green, spazzy, newly 5yr old (first time I trailered and rode him at a different place) but oh my did that arena scare me!
Are you planning on putting something on the sides to make them safer? My neighbors have kindly offered me the use of their arena this winter but since my choice was near decapitation or scary barrels and chairs... I'm not sure I want to!