Keigo Moriyama on Emulsive explains How to shoot Instax Square in an Instax Wide camera:
One of the characteristics of these [instant film] cameras is that we can choose only one format for one camera.
[…]
The adapter design is a really simple one. Just a few rectangular shapes extruded here and there to fit in both wide and square format.
Keigo then demonstrate the prototype in a video:
It’s interesting to see all the hacks around Instax cameras.