For decades, having to turn pages while I'm trying to play has been a pet peeve of mine. I've long been experimenting with different schemes of carefully balancing pages, using twin stands, or stand extensions. This is the simplest, most effective, and most cost-effective idea that I've come across.
So I'm very pleased with this simple little thing.
It's just a flimsy piece of card with a couple of creases and an elastic threaded through it. You could make something along the same lines yourself if you wanted. I haven't tried it outdoors but I suspect it would disintegrate if wet and would flap about in a breeze.
The bottom ledge doesn't always sit flush on a stand. I find magnets very useful in this respect: magnets complement and stabilize this board very nicely.
Folding it up, you need to remember to open the bottom ledge first (otherwise it may deform). Even folded, it may not fit in your gig bag.
Turning many pages at once takes longer than turning a single page.
5 loose A4 pages are supported. 6 A4 pages tacked together works well, with a central hinge and optional outer retainers provided by the elastic. Combinations of attached and loose pages, or multiple sets of attached pages, are also accommodated.
The elastic is cleverly threaded so as to attach the board to the music stand and to act as either a hinge or a retaining band for the pages.
In short: simple but effective!