Mirror option for projector.
The location of the projector acts as the plane position for the mirror, in advanced options you can check or uncheck which axis to mirror.
Mirroring a complete blueprint has been suggested before and has been pointed out that it may be error prone.
I'm suggesting mirroring on the fly pretty much the same way that mirroring in creative works only the mirror is added to a visible projection, allowing space engineers to check the projection for correctness occasionally but (and critically) viable for survival mode.
Upon completing a build you can choose to save either half or the complete piece to blueprint, allowing for more freedom and efficiency in modular style build planning in survival and even creative.