Avatron Software: Air Display
Six quid for this, but I had to give it a go (two screens aren’t enough!). Feels like magic: a wireless screen! But updating (eg, dragging windows) is pretty jerky and it can use 10-25% of my MacBook’s CPU when I’m dragging stuff on it, 15% if something is updating frequently (like Activity Monitor). Around 2% when idle. But could be very handy for some things.