We tried "partial" blockers as well....no good. We shut off the flow entirely using the Home Depot adjustable vent....stays plenty warm in the BR and we do get extra flow in the forward room....we do however, continue to use a thermostadt regulated space heater in the front when temps are below 32 degrees
...that o/s front cap does not seem to hold warmth in and the slide room up there certainly isn't insulated enough for the space involved. I am sure there are many improvements one can make....
. Good luck on finding satisfaction.