I used 16mm welding cable for all my battery leads which is finely multi-stranded so flexible and comes with black insulation so is not an eyesore and is easily capable of handling starter current draw.
https://www.ebay.co.uk/itm/Copper-We.../233285626643?
https://www.ebay.co.uk/itm/Copper-Tu...ter-Ring-Crimp