I felt uncomfortable with the Ender 3 fan being open to bits of filament etc. dropping into the board enclosure, as it's open and draws air in whilst running. Plus the fan draws air in directly under the heated bed. This shroud means the top is covered, making sure that debris doesn't fall in, and the incoming air is pulled from the front, not from the base of the bed. Despite the Ender 3 having probably the best (in terms of adhesion) surface I've ever printed on, I have had a couple of prints lift at the front left corner. I can't say yet whether it is definitely the board fan pulling air on the underside of the bed, but I can't imagine it helps.
I have included a step file for anyone to alter. If you want a different file type, feel free to ask.
Remove the 2 screws on the top of the board enclosure, fit the shroud, refit the screws.
Thanks to kennyboy for pointing out that it doesn't need supports if it's printed on it's end ;)
*Update: Added a version with fins running the length of the inside, so it'll print flat without support, and looks a bit better from the front.