slice params - depends on weight it should hold. You can add inputs to GPT and it will tell you recommended print parameters:

ME: “I want to print a shelf holder made of PETG material. The shelf is 15cm long, the supporting arm made of PETG material is 10cm. The second vertical arm of the support is 5cm. There will always be two supports for the shelf. The shelf will carry a maximum of 7kg. The width of the holder is 20 mm. What parameters would you recommend for the perimeter, filling and layer height for 3D printing so that the PETG shelf can hold 5-8kg?”

GPT:

• Perimeters: 3-4
• infill: 40-60%
• height of layer: 0,15 - 0,2mm (i used 0,15 - better for structural parts)
• infill type: triangle or gyroid (i used triangle)

I also added second .stl, half of a shelf. It has holes on both sides.