We tried to argue from our German experience, which mixes the constituency representation with proportional representation of parties and requires that a certain number of seats are available for the proportionality component.
If these seats are not available, then I simply cannot see how proportionality can be injected in your system—not even in a low dose. If all seats are filled via constituency plurality, or even more constituencies, I wouldn't know how to do it other than increasing the size of the House in Parliament. If there are no seats available for proportionality adjustment, then I have run out of ideas of what we could suggest.