Ok, if you'e running 10 min mains, it will be worth (as Kev says) just getting a fan for very little money, rather than investing in a brand new speedo.
Just wire it up to your power wires and it will be on when you plug in, cooling it down before you race, then cooling it off when you're marshalling...as long as it isn't unplugged.
Worth a try before you spend some mega bucks!
Also if you haven't already, cut holes in the front and the back of the little intakes too. This way its blowing the hot air out of the back of it rather than just circulating the warm air inside the shell.