The iPad 3 Retina display with a custom driver board is probably the fastest LCD screen out there for MiSTer use (less than a frame of lag), is native 4:3 aspect ratio and its 2048×1536p resolution is natively supported — and great for scaling most of the console and computer core resolutions.
It used to be that you had to buy a driver board and a case and assemble it yourself, but there are now pre-assembled versions available. It even comes with speakers built in, and a headphone jack!
I recommend the mini-HDMI version unless you need VGA (which you don’t with MiSTer), since it’s quite a bit thinner. The VGA version is 22mm, the mini-HDMI is 13mm.
Update: There is now a USB-C powered version available too.
Oh, and do not use the included mini-HDMI cable, it isn’t capable of handling the full resolution, get a high-speed HDMI-to-mini-HDMI from a reputable supplier instead — I use the AmazonBasics one, works fine.
I use it as my desk and travel setup, the stand is an iPad stand I already had available — but it also supports VESA mounts: