Mike, most of us use the H-shaped rubber intended for the Microbus engine tins.
I say most of us ... I mean those who, unlike myself, have cars where the engine compartment is where it's supposed to be.
I used to have it ... ;)
As for the tins, I have had very good luck with a pair of tin snips and roofing tin. The last one I did involved using posterboard. I traced the edge of the tins attached to the engine on the posterboard, then used the edges of the same square box you referenced for the outer edges. If you plan on cutting three pieces of tin, you'll do well to have one for each side, going from the front of the engine compartment to the rear apron, and a third piece which makes up the space between them. You'll be done in about three hours, start to finish.
In the photos below, the tins on the green car were made as I just described; the H-channel rubber is not being used on that car. Instead, it's a rubber C-channel, edge trim piece from an RV which has an aluminum channel inside. The aluminum has been crimped onto the tins. To do that, the tins have to be ever-so-slightly bent up or down at the very edge -- about once every six inches or so -- in order to keep the trim piece from coming off over time (due to vibration).
Any piece of decent-grade metal will work. My preference -- what I used on my own car -- is sheet aluminum. Believe me, if I can do it, anybody can.
Welcome to the Madness!
