The dark gray R1EM Rear Lens Cap from Sony is an excellent way to prevent dust and scratches from degrading the rear element on E-mount lenses when they are not attached to the camera body. It also guards against incidental bumps that would damage the rear element.
ALC-R1EM Rear Lens Cap for Sony E-Mount
SKU: ALC R1EM RearCap
2.7 x 2.4 x 0.4"