Custom STL Design for Plastic Spacer

I need an STL file designed for a plastic spacer measuring 3.5 x 6 x +/- 0.25 inches. The spacer will be used for mechanical purposes, and it should be solid with no holes or cutouts. The design features opposite faces that are curved with two different radii.

Ideal Skills:
- 3D Design
- CAD Software Proficiency
- STL File Creation
- Mechanical Design Experience

Please provide examples of similar projects you've completed in your proposal.