Homomorphism from $(GL(3, \mathbb{R}), .)$ to $(\mathbb{R}_{>0}, .)$ such that $\Phi(2I_3)=8$

Question

I need to find a Homomorphism from $(GL(3, \mathbb{R}), .)$ to $(\mathbb{R}_{>0}, .)$ such that $\Phi(2I_3)=8$.

I know that $\Phi(A)=|det(A)|$ would be a homomorphism from $GL(3, \mathbb{R})$ to $\mathbb{R}_{>0}$, but I don't see how to modify this to be a homomorphism that satisfies the second condition.