The overlays generally encompassed only the top row of the keyboard (which is an island) right? So they indexed the top row and the row immediately below. At least those are the kind I remember and are easy to find on Google images. Or were there also larger/full keyboard overlays that made use of the other gutters between islands of keys?

Also, I remember these for the Model M keyboard layout. What other standard keyboard layouts were popular for the cardboard guides?

Since everyone followed the IBM keyboard layout, the templates fitted virtually every keyboard. Right now I'm typing on an Apple Extended keyboard from about 1987 and it has little pegs at top right and top left corners of the casing for the keyboard templates to hang on.

Before the Extended layout, the F-keys were in a vertical double row to the left of the QWERTY block, as shown here: https://deskthority.net/wiki/IBM_Personal_Computer_keyboard

There were some F-key cardboard templates to fit over those keys, but they were less common -- partly because the original PC was so limited, maxing out at 640 kB of RAM and a 4.77MHz CPU, and not coming with a hard disk as standard, just 360 kB floppy drives, so that apps tended to be small and simple and not have very rich functionality to learn and remember.

But 3rd parties made ones you could stick on, e.g. https://imgur.com/gallery/pwNQZ9S