Used the two great resources (thank you guys!) and adapted it a bit:
1) 1. Layer has a larger surface to allow better adhesion to the printingg surface
2) Supports are not needed. The supports needed for the Joy-Con locking are already included in the model
3) Back of the grip is now solid from top to complete bottom. Before, there was an edge that just did not feel right for me, when holding the grip
If, after removing the support in the Joy-Con locking mechanism is not helping enough, to firmly hold the Joy-Cons (which it should), make sure that the upper edges are really horizontal and that the edge is not rounded by miss-printed layers.