Tow the freighter with your ship but tow it in a known direction and tow it further than the jump range of the enemy capital ship. Have a second MBR waiting at the position where the capital ship will end up. Tow the freighter 90 LY north. The capital ship will try to follow the freighter but won't keep up. The pirates nest.
Privateers have real trouble defending a specific planet against a large fleet.
The best defense is that they shouldn't know were the SB or rich planet is. But assume the worst. Your enemy knows where to do you damage and has a fleet on the way. Don't fight back. Set up a pirates nest at a planet they are likely to pass through.