I’ve found that reminding folks who use the “you have nothing to hide" argument often quickly change their mind if you just remind them the political party they don’t like could use it against them

This worked surprisingly well and without fail for the conservative side of my family. The instant I reminded them who was president at the time (Obama) all of them changed their mind

I don’t love this strategy. It leans into fear mongering a bit much, but if none of the other very helpful rhetorical techniques folks have shared here don’t work. This one has some staying power.