Keep in mind that the Instax Wide image area is of a 16:9 aspect ratio, not 1:1 square format like Polaroids.

As for the flash, you can experiment with diffusers over the flash to control output power and light quality. Even colored gel filters can be used, you just have to try it out.

Here's a sample image of mine:

Note the shiny hotspot on the left fender of the car has a small black spot, that's solarization that happens with the Instax film if grossly over exposed.