It might sound like I’m being flip, but I’m not — have you considered a sand bag? It seems like you could put a nice pillow cover over it if you were worried about appearances. (I would probably do that, if I played outside very often. 🙂 )

If the wind is so strong that it is blowing over a music stand outdoors (other than a cheap wire one that really isn’t suitable for gigs anyway), I would think twice about playing the harp in that area.

I don’t use a stand at all, but I’ve seen some players put a weighty sheet of see through perspex over the music to stop it from blowing off the stand.

I have a Wenger Port-a-stand. It’s portable but has a really heavy base. It also has two elastic straps to hold the music in place. I’ve used it for outdoor playing quite a bit and it has done well.

I also really enjoy the fact that it can go down to a short height. Great little stand: http://www.wengercorp.com/Stands/Portastand.html