I have the same problem....could not find an "inline shutoof", so I made one... got a couple of brass fittings from home depot, one with a shut off lever, cut the rubber hose to the heater box along the valve cover in a spot easily accessed , installed my shut off there...brass fitting a little heavy so I also installed a bracket that holds it away fro the valve cover, paint it engine color, and now I can totally shut off the heat...open the hood , open /close it..usually once or twice a year...summer /winter

I know ,its "rigging" it , but works well....dont have a pic available...but can take if wanted...
I`ll be with my car in an hour or so, I`ll take one , just incase...