Rectangle-Cylindrical Enclosure ideas?
As your objective appears to be the requirement to present a product, you aren't asking for production methods. This suggests that a suitable method is to use 3D printing, specifically resin printing, to create the enclosure.

3D resin printing is nearly layer free (strong magnification required) and presents a smooth professional appearance. It can be painted to provide the necessary coloration.

You'd be able to have an enclosure designed specifically for the components to be placed within, providing structural support for circuit boards, switches, LEDs, etc.

Online services such as Shapeways will create these prototypes or you may find local resources. A nearby makerspace would also be a resource.

Another alternative is to create a plug/mold/part system. Carve, cut, sand or otherwise create a duplicate of your design in any convenient material. Even balsa wood serves the purpose as long as the surface is sealed.

Pour molding silicone around the plug and when cured, remove it and pour resin to create your final component. Resin can be colored as required, rather than to resort to applying paint.

The above is a summary, as the process is slightly more involved than described, but is an option for you to have a reasonably professional appearing enclosure. Considerations involve resin pour path and venting, mold parting and alignment, etc. Additional research is recommended if this is your selection.