I’m pretty sure, somehow, Prince wasn’t actually talking about a beret that actually looks like a raspberry. Nothing that comes out of that guy’s mouth should be taken literally unless he’s speaking explicitly.
The Raspberry Beret Shower Cap does exactly that, taking the classic Prince song to the literal degree of crazy to the point of actually putting a literal raspberry-shaped beret on your head. For $8 you could certainly do worse. At least it’s not a Little Red Corvette.