I, too, did not do this the "CMC Way".
Rather than use stair tread material as a lower flap, I cut a piece of 3/8" thick shop floor padding (Home Depot flooring dept. - comes on a big roll and is 4' wide) cut to fit the shape of the firewall, attached it with 3-M spray-on adhesive and some noise blanket plastic mushrooms and extended it down to meet the cooling tin breastplate that crosses the rear (transaxle) end of the engine, then fastened it to the breast plate with the plastic mushrooms used to hold the sound blanket onto a water cooled car's under-hood area.
Just thought that whole "using stair-tread material" a bit too John Steele (and that was ten years before we even met!) and knew it could be done better. This way, not only does it seal the front of the engine compartment, but it cuts down on engine noise a little bit, too.