You can get a very small amount of adjustment if you loosen the 4 screws holding the metal strips in place. Then with the side screens mounted (only hand tight) you can try to push the side screen to the position you want. Best done in two so that you can tighten the screws whilst it is being pushed forward. This will not gain you much but it may be enough.
I have also bent the screens inwards at the top so that when the door closes there is pressure against the rubber.
This, in part, is the result of the inaccurate nature of the hand-built Morgan. Other than every door being different, they are made to fit the car, there will be a tolerance in the fitting of the windscreen pillars. A 0.5mm difference at the bottom of the screen could amount to 2 or 3mm at the top. The ultimate solution may be to loosen the windscreen pillar mounting screws and push it back a little.