Outdoor tiles require gaps. As our stone tiles are wider on the bottom, it wasn't easy to hold a good distance for all tiles.  So I designed these distance holders,

They are 2.5 cm deep, go 3cm in all directions. 

Gap Top: 1cm

Gap Bottom: 0.5cm

The two parts are a normal cross and a T-part for the edges.

Print:
Infill: more than 10%, you don't want to crush them.

Supports: No


Future plans:
.scad model to parameterize gap distances.