Wall-Mount Bracket 3D Design

I need a custom wall-mount bracket modelled so I can print it on my Snapmaker U1. The bracket’s sole purpose is mounting, and I will be printing it in PETG plastic, so the geometry should favour strength while staying practical for FDM: good layer orientation, filleted stress points, and minimal overhangs that demand support.

Scope
• Model a bracket that sits flush against a flat wall surface and accepts standard countersunk screws (please size holes for M4 unless you recommend better).
• Include a matching recess or boss on the outward face for whatever I choose to attach later; keep this area as a clean, planar surface 50 mm × 50 mm so I can modify it down the road.
• Provide the native CAD file plus an export in STL ready for Snapmaker Luban. Build volume won’t be an issue, but stay within 150 × 150 × 150 mm so I can rotate it if needed.

Acceptance criteria
1. No mesh errors; the STL must slice in Luban without repair.
2. Mounting holes align perpendicular to the wall face and are chamfered for countersunk heads.
3. Printed in PLA with 30 % infill, the bracket should handle a static load of 3 kg without visible flex (simple FEA screenshot or hand-calc welcome).

Let me know any clarifying dimensions you need, and include suggested print settings in a short note with the files.