Thats an excellent rule of thumb! You can also go to home depot or lowes and pick up some sheet metal (the cheap stuff roofers use around the base of a chimney and whatnot). Then make a prototype of the following template on a piece of paper or cardboard:
Cut on black lines, bend on pink. White is the material, and blue is background. Once you get the right form, apply that prototype to the sheet metal and cut away. Prime it and spreypaint it the color of your heatercore. When they are finished it looks rather professional for a cheap and easy mod.
Then use the thin window insulator foam strips between the fan and the shroud, and the shroud and the heatercore, and the heatercore and the case. You can also use it as padding for your pump or heatercore to sit on in your case. This prevents a heck of a lot of vibration noise. (found at hardware stores for a buck or two, use the rest on your windows or doors in your house
).
To attach everything together, I ran 4 long zipties starting from the fan, through the shroud, through the heatercore, and through the case chassis. Then I used the ends of 4 extra zipties to attatch them to the case, and ensured they were tight. This is the cheap, ghetto, but effective way to make sure everything is secure together.
The gladware shroud is much easier, but it looks too much like a gladware container in the end
.