Depending on how you are wiring up whatever you need to, have you thought about non-split convoluted conduit, and nylon gland nuts? The conduit comes in 12mm 16mm 20mm 25mm etc, and the gland nuts are sized to fit them. You can also get 'T' pieces and adaptors, so that you can build a complete harness with branches, if that suits. That's what we used on mine service vehicles, to install all the lights and accessories. The conduit and cable glands makes it a waterproof system, which you wouldn't have just passing the wires through the glands.