Your overall method is right but I don't believe you've simplified the boolean algebraic expression quite right.
I recommend first trying to implement the Mux without simplifying the big 4-part OR expression.
Another thing to think about is that "when sel is 0 it doesn't matter what b is"; and "when sel is 1 it doesn't matter what a is." If you think about it, then there are really only two cases where the out has a meaningful 1 output.
Give it a shot. If you get the non-simplified version to work, I'll post the simplified algebraic expression.