A fully parametric SCAD file to generate your perfect fit outdoor lighting mount. there are comments in the script for how the parameters work. I uploaded some samples that the script is capable of.
Things you can change:

• total shape: length, width, height, outer edge chamfer
• shaping: peak to peak and section depth to match your siding or none for a direct to wall mount
• cutout: size, shape, x/y axis offsets, or none for a flat solid surface
• screws: size, inset size and depth

I used it to custom fit every outdoor lighting mount to match the hack job vinyl siding cutouts so I didn't have to caulk hide the gaps :)

Bambu Lab

Bambu P1S

No

No

25%

I highly recommend printing prototypes to get a perfect fit: use 2 layer bottom, then 1 for walls and top, 5% infill.

When you're happy with your fit, I recommend these settings: 5 for bottom and top, 6 wall loops and 25% infill. I used triangles. You want compressive strength since you're going to screw it in place and it needs to hold up against that. Don't over tighten, no need to go crazy, the light will mount to the receptacle anyway this is just a fancy spacer.

If you are using it as a full mount, then up the infill and make sure your wall loops are enough for your screw holes.