Joe, I too did something similar to the other guys, but maybe even a bit simpler and possibly more along the lines of what you’re thinking. No cutting or modifying of the box required.
Once the evaporator was removed and the heater core replaced, I fabbed up a really simple cover with an inside bend out of light gauge sheet metal. Trimmed it to match the shape of the box around it, painted it to match the a/c box, and urethaned it in place. Keeps the water and elements out, and seals up the box so you don’t have a parasitic loss in the fan efficiency.
Hope this helps.
D.
Once the evaporator was removed and the heater core replaced, I fabbed up a really simple cover with an inside bend out of light gauge sheet metal. Trimmed it to match the shape of the box around it, painted it to match the a/c box, and urethaned it in place. Keeps the water and elements out, and seals up the box so you don’t have a parasitic loss in the fan efficiency.
Hope this helps.
D.