Exporting a slice in Sketch with transparent background around

Using Sketch 3 want to export an icon, the icons themselves are 24x24 but they sit within an area of 32x32.

I've figured out how to do it by combining two different layers one 24x24 and the other 32x32 but it's a tedious process. I need to do this for every single icon.

Is there a faster workflow?