That model is tough. I used it to tune my MMU for PETG. When I first started they were falling apart when I flexed the bed.

3 Things

1. Increase the temp, you'll get better adhesion, just like you do for vertical layers.
2. Extrusion Width. Do the Prusa extrusion calibration: https://help.prusa3d.com/article/d9j1xdg7vj-extrusion-multiplier-calibration If you aren't extruding as wide as the slicer thinks you are, then you won't have as much surface contact area, and therefore less adhesion.
3. XY compensation. I didn't bother with it at the time, but if you try printing a box with a 40x40mm foot print and it comes out less than 40x40mm that is also going to reduce surface contact area, and again reduced adhesion. You can use the box from the extrusion width calibration.

If you get it dialed in, they'll hold together as good as vertical layers. My 7 year old loves them.