Mylar is quite common for wetscanning on for example an Epson flatbed. You’ll see it used in a few youtube videos, and Scanscience supplies mylar sheets if you buy their kit. Getting it flat isn’t a problem when using fluids, at least.
Film holder – a negative carrier from an enlarger. That’s what I use.
I just used a glass over my iPad (1st Gen) which worked really well but have to use some kind of diffuser cause you see the pixel grid big time in the final scan…
Has anyone used this smartphone scanner to scan mounted slides ?