The best option for 3D printing in near-transparent plastic is SLA Accura ClearVue, Quick Clear finish. This auto-quoting option prints the part in a clear resin then has a post-process treatment of a clear coat (paint) to even the surface and leave a gloss clear. Think clear ice as a finish. The video below shows some Quick Clear parts.
Although they are not optically clear, we have several materials that can hit different needs from light-pipes to case bodies. Stereolithography (SLA) is a go-to for transparent or translucent parts. Clear SLA options include Accura 60, Accura ClearVue, Somos WaterClear Ultra 10122, and Somos Watershed XC 11122. SLA parts will have an icicle-like translucency on unsupported features but are matte on features that require a sacrificial support structure. Note that the default finish is matte, so standard or strip-and-ship finishes should be selected if natural translucency is required. SLA parts can be post finished or polished for higher levels of transparency.
Video: Clear 3D Printed Parts
Our PolyJet 3D printed Translucent Rigid Photopolymer (VeroClear) has a frosted translucent finish which can be polished. We also offer FDM 3D printed ABSi, which has a diffused translucency in tints of natural, orange, and red (such as car tail lights). FDM PC-ISO is also a translucent filament in it's natural state.
Photo Examples of Translucent Materials
(The left photo demonstrates a VeroClear material in PolyJet 3D. The right photo demonstrates translucent FDM materials with ABSi in Orange and PC-ISO in Translucent Natural)
Ultimately no 3D process can make a perfect transparent or translucent solution out of the machine without significant processing. Milled materials like clear polycarbonate can sometimes be a better fit but will have milling marks on any subtracted surface (they look like small swirls). Xometry can review to make a clear model through a significant amount of manual labor but often the price can be an order-of-magnitude higher than the material quote.
Photo Example of a clear polycarbonate which has been machined by milling.