This 5 gallon Optimum Polymer Technologies branded bucket fits the grit guard insert perfectly. Made from natural (semi-transparent) sturdy plastic.
Incorrect washing is almost the one and only source of swirl marks and scratches, caused by dirt particles trapped between your wash mitt or sponge and the painted surface.
If you intend to prevent scratching the surface you are cleaning, yes you need a Grit Guard insert or something equivalent. While no one tool or accessory will guarantee that you won't damage your paint while working on it, they will guarantee a higher chance of not harming your paint when used together.