How best to strain relieve cable entering aluminium enclosure
You can use a cable gland.
For your use case:

Select a cable gland which can hold a 6mm cable. These glands can either be made up of metal or nylon, like the &quot;RS PRO PG 7 Cable Gland&quot; or &quot;HellermannTyton NGM M12 Cable Gland&quot;. At any rate, these should provide strain relief and at least IP67 rating -- you turn the gland with the cable passing through and the rubber or nylon material inside squeezes on the cable.
You need to drill a hole larger than 6mm to accomodate the gland. The specific size depends on the gland that you'll be using.